KySilverado

iTrader: (7)

It sounds like an angle changing to me. Springs getting weak and possibly axle wrap at take off. Doesn't really address the 55 mph steady cruise. Another possibility, you mentioned the rear tranny mount. I'd look at changing that out. Under torque if it moves around it would change drive angles and possibly vibrate. If a 2 piece shaft look at the carrier bearing and mount. Check the pinion bearing for slop.