My problem turned out to be loose suspension components up front. The shop tightened everything up and that fixed the shake. I would check everything specifically the spindles.

 

Still sounds u-joint like, but I'll throw this at you, how are the brakes? Also, the ebrakes? I say this cuz maybe a caliper is sticking, or a warped rotor. You said the tires are balanced, where's the shake felt, in the seat or steering wheel?

 

my 2003 4x4 has always done that to me. shakes at 40-45 and 80-85. ive taken it to a few places over the years and they all thing the tires just need to be balanced but that hasnt fixed mine yet.

 

Now that I think of it, check tie rods and that kinda stuff. You'd feel that in the wheel.