Ok in 4th gear running 40-50 mph i have a vibration.
u joints do have a little play and really greased them 200 miles ago.I slip the tranny in neutral and alot of it goes away.Just seems under load and once in 5th doing 55 and up it feels fine.Any ideas before a u joint replacement?

 

I'd say replace them. Good news if the vibration was not in 4th gear but occurred in any other gear it could be the input shaft on the transmission. I just went through that fiasco. Good luck

 

It doesnt seem to do it any other time and once i shift to 5th and get to 55 it quits.Did your input shaft have alot of play?

 

just an update.Pulled rear drive shaft,front u joint bearing surface where chaffed.
noticed a leak.Pulled the splined hub off of transfer and the teeth are worn and leaking
past the grooves.I guess that would cause a vibration.Got another one coming.
The output shaft looked good...