1987 Nissan Truck

Shakes or Wobbles problem
1987 Nissan Truck 4 cyl Four Wheel Drive 230000 km

have a vibration between 65km and 85km, tried tires, shocks, balance drive shaft (hose clamps), changes under load and bumps, feels like a bent rim but it's not, ran it up on hoist rear drive shafts seem true wheel and axles run true, u joints move freely, pinion no noise or movement, center bearing good and has been replace before, but noticed not much movement available to compensate for drive line length movement, know two other people with same problem, however shocks did improve the problem a fair bit, someone told me that these trucks have weak rear springs that cause it to bounce, have you ever come across this? Thanks Axle