Just got 71 K5 home last night! HEEELP!

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I haven't even gotten under it yet, but I had to drive it 90 miles last night and there is what I would consider a severe vibration in the drivetrain somewhere. The reason I know that is when I let off of the gas and coasted, it stopped vibrating, and as soon as I got back on the gas it would persist. It has a 350/TH350/NP205 to a 12 bolt rear w/ 4.10:1 posi.

My first instinct is:

Driveshaft unbalanced
U joints toast
Play in pinion bearing?
undesireable driveshaft angle (4" suspension lift)

The gd owner didn't say ONE word about this. Plus it didn't have seatbelts and the seats were not completely bolted down. SCARY ride!

Anyways, I love the truck, but it isn't even driveable right now. Plus I am nervous, because with this much vibration, I know it has transferred to the transmission and crank over time.

Any pointers?

Thanks!

Russell

Pics soon!