Since you think you might have the driveshaft in "backwards", I assume you replaced the u-joints. If so, that should rule them out. You're on the right track by eliminating things, just need to do it one item at a time. Possibilities include:
unbalanced driveshaft
pinion bearing
tranny tailshaft bearing
front yolk to tailshaft spline wear
pinion bearing - differential
rear axle bearing
tire balance - even though they were just spun, it's still a possibility, try swapping them.
I'd start by finishing your driving test. Assuming the vibration is still there, I would crawl under with a prybar or huge pliers, wheels blocked with weight on them. See if you can find movement at either end of the shaft. This will check the u-joints and bearings. Then put the rear end on stands, tranny in gear. See if the rear wheels have any side to side movement. Check the front u-joint and tailshaft for slop. If you have a dial indicator and stand you can check the driveshaft for straightness on the truck.
If all this has failed, I would then go to a driveshaft shop, and I hate paying someone to work on my truck.
