1989 Other Chevrolet Models Differential or Clutch

1989 Chevy 1/2 ton 2wd. Vibration/shimmy that is noticable at 30mph. Comes and goes, but noticable again at 60mph+. First thought was tires, really feels like a tire issue but I have brand new tires which I road force tested and re balanced and are perfect. I have replaced worn u-joints. Inspected all suspension components. Everything checks out. This vibration IS NOT felt on a rack, only driving. Is it possible a broken clutch spring(s) may cause a bad vibration only while driving? Is it possible that exessive gear lash or pinion play in the rear end can cause a bad vibration only under driving? (I do have a pinion bearing set that does need installed, its noisy but no vibration)THANKS