I had the same problem with my Pro-Comp's and it drove me nuts. they balanced perfect, but when I raised the truck up and let the tires spin freely, 2 of them were visibly out-of-round.

I ended up having to shave the tread to get the tires round, but it only works temporarilly.

After about 5,000 miles, they are out of whack again because the rubber was so uneven around the circumference of the tire.

 

check the steering linkage, tre's, ball joints & tracbar. if you yank the driveshaft, get it rebalanced. they suck from the factory. had a friend do his recently because of bad joints & his was also out of balance.

also check the alignment... caster should be at 5.0 or greater. if there is not enough caster it will act like what you describe... ask me how I know...

 

its the rear drive shaft..... i had exact problem on mine at 33000, and at 65,000.... it was rear drive shaft both times. both were factory shafts and both twisted. had exact same vibrations your have and checked everything that moves on truck first time cause it was warrentee. second time just pulled the shaft and had a custom made and vibration gone.