98 GMC extended cab drivetrain vibrations

I have an 1998 GMC extended cab (2 wheel drive,350 engine) that started giving a vibration at around 45-55 mph. It does not shake the truck, it is an audible “drumming” sound inside the cab that is very annoying. Here are the things that I have done to try to correct it.

Replaced all U-joints, removed the rear shaft and had it checked for straightness and balance.

Bought another short driveshaft (front). Replaced one of the axles,(one looked like it may have been bent).

None of these made much if any diffence.

I also unhooked the driveshaft from the rearend and tyied it up and pulled the truck with another truck up to about 75 MPH and I could not hear it vibrate. This to me eliminates the rearend, unless it is doing this under a load.

I got this second hand and had it several years prior to it starting to do this.

Any help would be appriciated.

If this is a two piece rear driveshaft, the problem might be in the hanger bearing.


Forgot to add that, it was changed too.