'88 BroncoII 2wd 2.9 Loud vibrations at 65-70mph

[VeeSixer]

I have a 1988 Bronco II 2wd 2.9L that i bought about 8 months ago. i just had the tranny rebuilt and the driveshaft checked and everything appears normal. But i've had the same problem since i bought it where the truck will begin to vibrate with a very loud, low hum once i get to 65mph-70mph. the thing is that if i accelerate through that region without letting off the gas, the hum never really gets loud. it kicks in the moment i let off the gas. you can especially feel it through the gear shifter (it's an auto tranny so whatever you call that stick). i've driven it at least 30,000 miles since i bought it, mostly highway miles, and it hasn't lead to any major catastrophes, i just want to get it fixed before it does. any suggestions on where to look? or link to another thread dealing with this same problem? thanks for any help,

cheers

 

[Ranger Dave]

check the u-joints and maybe the torque converter nuts