if the vibration was near the same and at same speed after two separate tire balances than I dont know if I would bother looking at the tires much more. The only other thing to look at would be if the wheel or tire is out of round. In that case you can actually trick a machine into balancing it but it still will not make it round again if you get my meaning.
What this really sounds like to me is drivline angles or as mentioned above the driveshaft being out of balance or out of phase. That is where I would start... pull the driveshaft and take it to a competant shop to check for balance, phasing and correct length. Next, while the drive shaft is out you need to check the trans tailshaft and the pinion yoke for correct pinion angle... my money is on bad pinion angles