As Killerblackbird previously mentioned, being out of round is a very common problem that dealerships and tire shops often overlook. It feels exactly like the tires are out of balance and shake only at certain speeds. They can balance them all day long and they'll still shake.
To check your tires, jack the tire slightly off the ground leaving about an 1/8" gap and rotate the tire. If the gap changes, they're out of round. I know that BFG's are famous for this problem. My tire shop mounted 14 brand new tires on my truck and I still only managed to get 3 good ones. The tire shop owner says that its a common issue on larger sized tires, and especially BFG's.