hi
im not sure if it would have a 1 or 2 piece driveshaft. i know the short bed dodge trucks around these years used a 1 piece shaft, not sure about the long bed trucks. those may be a 2 piece.
i had a 71 chevy longbed half ton C/10 years ago with a built 350/muncie m-22 4 speed combination, it also had a 12 bolt positraction with 4.11 gears. this ***** would run, and run hard. it had a 2 piece driveshaft, and would shudder and shake pretty bad till i changed out all 3 u joints, and installed a new hangar bearing. then it was pretty smooth after that.
if i knew how to weld back then i would have beefed up the frame where the trailering arms attached to the frame (coil spring rear) then i would have cut out the oval hole in the crossmember the drive shaft went thru that the trailering arms attached to and just would have had a 1 piece driveshaft made. reason being is i drove it like an animal , 6500 rpm launches by side stepping the clutch kept messing up hangar bearings. not the easiest thing to change.
if you can modify it to a one piece shaft, i would definitely reccommend it. if you cannot do this, then rebuild the shaft, new u joints, and hangar bearing, then get it balanced at your local drive shaft shop.
axle pinion angle set incorrectly with a 2 piece driveshaft will cause shudder at different vehicle speeds and driveshaft RPMs, it can also move around to different RPMs and vehicle speeds depending if the truck bed is loaded down or empty if the pinion angle is incorrect. since the driveshaft changes angle in relation to the axle depending if the truck is loaded down or empty.
ran into this problem with a 1960 chevy el camino X frame full size car. it uses a 2 piece drive shaft, and had shims to adjust pinion angle to reduce driveshaft shudder. trial and error was the only way i was able to fix this.
it also may be as simple as a blown out transmission rubber mount. this will cause a lot of vibration too and is an easy fix.
vehicle harmonics what a *****.
i hope this helps you out
matt
