Rear main seal leaks..does this affect transmission??

Oil leaking around a starter is usually one of three possible things.
If it is motor oil, then suspect the rear main seal.
It it is auto trans fluid, it could be from the transmission front seal or the torque converter
If you have a manual transmission, it could be clutch fluid leaking from the slave cylinder, or a leaky front seal from the transmission.

black oil or red oil
if red it is a torque converter seal and the transmission will have to be removed
use a professional auto transmission specialist shop
black oil is sump oil and can be from a rear main seal or leaking from a rocker cover gasket and running down to under the bell housing making it appear as a rear main seal

Try running some of that high-mileage engine oil next time you do an oil change (it helps to swell engine seals slightly). If the problem clears up or is reduced, then it's likely that the rear main oil seal is starting to leak a little. The rubber seals will harden up and shrink a little with age.
I would also powerwash the entire area clean, so that way any leak will be easier to monitor and pinpoint.

1. Are you losing oil? Have you been told this seal is leaking? This seal is located between the engine and transmission connection. This seal will be soaken wet with oil if failing. Your motor is probably the GM 3800 V-6...this motor is not known for excessive rear seal problems. Hope this helps.

the oil reservoir could have damage. check for the other oil line.

List of possible locations for engine oil leaks:

Valve cover gasket
can,crank and oil pump seals.
rear main seal
oil pan gasket
Transmission oil leaks
Vents, dipstick,torque converter ( need to examine shell to pin point whether its the oil pump or a converter seal)transmission gasket,axle seals, drain plug.

First have to determine where are you loosing it from.
Possible engine oil leak areas : Valve cover gasket,oil pan gasket, rear main,oil filter. crank or cam shaft seal, oil pump. Pcv valve.
If its transmission oil leaking here are a few areas of interest: shell of transmission( depending where on the shell could be oil pump, or converter., vents, and axle seals.
If your not capable to jack the vehicle up and look for the leak , then take it to a repair shop.
Keep me posted

this truck is equipped with an aluminum oil pan you could have 1 or several things going on... a stripped drain plug which is causing it to leak or it may be a leaking front seal or a rear main seal. If it is a front seal it is not to labor intensive. But if it is a rear main seal leaking it will require the transmission to be removed since the seal is a 1 piece rear main

First I'd find a different shop. $100 to look at it? I don't think so. Sounds like this may be a transmission issue.

Could be quite costly. Especially the rear main seal on the engine. You have to take the trans out to even get to it. I would look for something else with less problems to buy.