How do you replace the rear engine seal on a 84 c1500 350

not told at all which end leaks (the exact spot matters)
I bet there is lots more wrong !
when did car last run good, or not leak ATF
last week,, last year, last decade? what?clues help.

history matters on all and service done. for 39 years
last owner left the pan loose and 2 bolts left and 5 turns loose, we cant see under any car.
front seal blown on or behind TC
rear seal; blown
pan gasket a wreck
pan hit rocks and cut and leaks
side shifter rod seal blown
or sensors on it leak (VSS)
governor cap seals blown./
more, sure.

camshaft sprocket bolt 58-72 ft. lbs. crankshaft pulley bolts 108-132 ft. lbs. crankshaft pulley center bolt 80-94 ft. lbs. cylinder head bolts cold engine 65-72 ft. lbs. warm engine 73-80 ft. lbs. cylinder head rear cover bolts 69-86 in. lbs. flywheel/Driveplate bolts 94-101 ft. lbs. intake/Exhaust manifold nuts/Bolts 11-14 ft. lbs. jet valves 13-16 ft. lbs. main bearing cap bolts 36-39 ft. lbs. oil pan bolts 51-69 in. lbs. oil pan nuts 43-60 in. lbs. oil pan drain plug 33 ft. lbs. oil pump cover bolts 72-84 in. lbs. oil pump relief valve plug 29-36 ft. lbs. oil pump sprocket nut 25-28 ft. lbs. Rear oil seal retainer bolts 84-108 in. lbs. rocker arm shaft bolts 14-15 ft. lbs. rod bearing cap nuts 23-25 ft. lbs. silent shaft sprocket nut 22-28 ft. lbs. spark plugs 15-21 ft. lbs. thermostat housing-to-Engine bolts 86-114 in. lbs. timing belt tensioner bolt 32-40 ft. lbs. valve cover bolts 43-61 in. lbs. water pump-to-Engine bolts alternator brace bolt 14-20 ft. lbs. all others 9-11 ft. lbs. water pump pulley bolts 72-84 in. lbs.

main bearing cap bolts 36-39 ft. lbs. oil pan bolts 51-69 in. lbs. oil pan nuts 43-60 in. lbs. oil pan drain plug 33 ft. lbs. oil pump cover bolts 72-84 in. lbs. oil pump relief valve plug 29-36 ft. lbs. oil pump sprocket nut 25-28 ft. lbs. Rear oil seal retainer bolts 84-108 in. lbs. rocker arm shaft bolts 14-15 ft. lbs. rod bearing cap nuts 23-25 ft. lbs. silent shaft sprocket nut 22-28 ft. lbs. spark plugs 15-21 ft. lbs. thermostat housing-to-Engine bolts 86-114 in. lbs. timing belt tensioner bolt 32-40 ft. lbs. valve cover bolts 43-61 in. lbs.

'84 ...*lotsa* miles on it ....like 450K plus if you drive the "normal" 15K/year. First, and most likely, thing that comes to mind is the wheel bearing has seized. Since this is a front wheel drive car, there are sealed roller bearings for the rear wheel hubs. At 450K on the odometer there are *lots* of components that have reached or exceeded their normal service life. I would suspect the left rear sealed wheel bearing is one of them ...followed closely by the right rear wheel bearing. If it's the bearing? Get them both done.

Four cylinder or V-6.

Both engines will require the removal of the timing belt to service the front crank seal. Both engines will also require the removal of the transmission to replace the rear main seal

either way way you would be looking at 3-4 hours for the front seal and 5 to 6 hours for the rear seal.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Have you washed down the engine, and ran for a few days to see if leak still persists, is it a flowing leak or slow leak, and are you sure that it is engine oil ????

If you have a shop you need to pull the engine to replace the rear main seal. There is a possibility of a two part seal. If you have a two part seal drop the oil pan and pull the rear main engine cap off and then replace the seal. You may need specialized tools.

I'm not sure if this is the right part of your car but, are you talking about the rear seal on the engine for the crankshaft?

If so:

Rear Main Oil Seal REMOVAL & INSTALLATION Fig. 1: Rear main seal installation for the 2.3L and 2.5L engines
Fig. 2: Removing the rear main seal
Fig. 3: Installing the rear main seal on the 2.9L engines
If the crankshaft rear oil seal replacement is the only operation being performed, it can be done in the vehicle as detailed in the following procedure. If the oil seal is being replaced in conjunction with a rear main bearing replacement, the engine must be removed from the vehicle and installed on a work stand.

1. Remove the starter.
2. Remove the transmission from the vehicle, following the procedures in Section 7.
3. On a manual shift transmission, remove the pressure plate and cover assembly and the clutch disc following the procedure in Section 7.
4. Remove the flywheel attaching bolts and remove the flywheel and engine rear cover plate.
5. Use an awl to punch two holes in the crankshaft rear oil seal. Punch the holes on opposite sides of the crankshaft and just above the bearing cap to cylinder block split line. Install a sheet metal screw in each hole. Use two large screwdrivers or small pry bars and pry against both screws at the same time to remove the crankshaft rear oil seal. It may be necessary to place small blocks of wood against the cylinder block to provide a fulcrum point for the pry bars. Use caution throughout this procedure to avoid scratching or otherwise damaging the crankshaft oil seal surface. To install:
   
6. Clean the oil seal recess in the cylinder block and main bearing cap.
7. Clean, inspect and polish the rear oil seal rubbing surface on the crankshaft. Coat a new oil seal and the crankshaft with a light film of engine oil. Start the seal in the recess with the seal lip facing forward and install it with a seal driver. Keep the tool, T82L-6701-A (4-cyl. engines) or T72C-6165 (6-cyl. engine) straight with the centerline of the crankshaft and install the seal until the tool contacts the cylinder block surface. Remove the tool and inspect the seal to be sure it was not damaged during installation.
8. Install the engine rear cover plate. Position the flywheel on the crankshaft flange. Coat the threads of the flywheel attaching bolts with oil-resistant sealer and install the bolts. Tighten the bolts in sequence across from each other to the specifications listed in the torque chart in this section.
9. On a manual shift transmission, install the clutch disc and the pressure plate assembly following the procedure in Section 7.
10. Install the transmission, following the procedure in Section 7.

the first thing you do is check the wiring to the sender, they may have damaged it when the engine hung down during the clutch change,

A leaking rear main seal will not cause your transmission to leak.

The transmission has its own seal and should not leak because of the engine rear main seal.

Due to the labor involved in replacing the rear main seal on the engine, it is best if you replace the front transmission seal at the same time if possible.

Installation varies on different vehicles but the principles are the same.