Use a straightedge on it. I bet you see daylight under it.....

Had the same issue on my van when I bought it.....

 

The driveshafts have to be in correct phasing...that is when you take the 2 halves apart mark the halves with a straight edge or chalk or something so they go back together correctly and keep the balance...also a weight could have been lost...
Take the unit off, take to a driveshaft shop and have them balance it as an assembly...
Other things that could cause problems...brake drums bent/missing weight/s, rear pinion bearing failing, rear mainshaft bearing/bushing failing/failed.

I would put the rear end on stands with wheels off the ground and put it in gear and watch what it does in the various areas...u joints f/r, center support, trans mainshaft at the rear...if the front yoke is moving that would indicate this issue.
The same as the rear pinion yoke would be the same culprit...bearings...
I had my carrier bearing replaced at a local shop and balanced...not a problem with mine since...
run the truck also with the rear wheels off...see what does/doesnt happen to the vibration...


- cs65

 

Ok I found the problem.. I put the truck up on stands and had a friend of mine sit in the truck and put it in gear.. I watched under from the back end..The back drive shaft is messed up. It was moving like a bent stick side to side...Im going down as soon as this rain passes and get another drive shaft.

I want to thank you all for the help... Cause if it wasnt for all of you I wouldnt know what to do..Thank you all again..

 

I would keep the front half as a spare...and the U joints...take some pics of it after you get it out...check the rear yoke to make sure it doesnt move in excess...would most likely be bare minimal movement ideally.


- cs65

 

Do yourself a favor and splurge on a retubed shaft from a driveline outfit. Used shafts, IF you find one, could have the same issue or worse.