I have a 2000 silverado 4x4 automatic with small v8. I've recently noticed a slight vibration with speed in excess of 30MPH and it appears as though my transmission is making a "clicking" sound when taking off. At first the clicking would go away when in either 2nd or 3rd but now does it when downshifting as well. Any help would be great...

sounds like a worn out transmission mount try rocking the ****** up and down and if the play in the rubber mount appears excessive this can cause all of the stated problems

The dealer is not going to replace something if they can not prove to the company the part is defective. The company won't pay the warranty claim.
A bent axle was my next guess. A garage could find out by using the same dial indicator used to check for a bent rim.
The dial gauge is placed against the shaft and the wheel is turned, if the axle is bent the dial will move.
I don't know what Road Forced balanced means, but most balancing machines duplicate road speed of about 30MPH. So a tire or wheel can balance ok on a machine but still have a vibration at speeds above 30 or 40mph. The easiest way to make sure is move the front tires to the back and see if the vibration travels too.
If you are describing the problem correctly, it almost has to be in the transmission, the axles, or the wheels. There is a slight chance a strut or strut mount could be faulty and allowing a small bounce to be amplified.

Did you lock the front hubs?

Have you checked the wheel balance on the front wheels?

Have the drive shaft checked out. You may have a bad hub bearing or bad CV joint. Look for cracks in the boot, and grease that has been flung inside wheel well

I think your balljoints in front are bad. On 4x4 I repaired recently, both of them where bad and it had a lot of vibration. I'm guessing it doesn't happen when it's only in 2wd.

sound like the u-joints in the driveshaft. Lay under the truck and where the u-joints are try moving the driveshaft up and down. If it moves then the u-joints are bed. This will cause a tremendous vibration under a load and the faster it spins it will smooth out.

check your universal joints

Check the u- joints are fitted in line, if ok check wheel balance both front and rear.

Have you changed the trasmission fluid and filter in your jeep lately, if not do so and that just might stop the jerking in your jeep, and if that does not do the trick you will need to have your trasmission checked out by a trasmission shop.