S10YODER

I have a 2003 s10, this has been going on for awhile but it seems to be getting worse. When travaling down the highway there is a loud humming noise that seems to me is coming from under the bed towards the back of the car. Its worse at 55, at 65 it kinda subsides. While driving in parking lots u can hear a rotational clicking noise coming from rear of vehichle. Today i put it up in the air on the rack and ran it at 55mph and i thought the car was going to shake its way off the rack. Then I removed the rear wheels and repeated that same process and the vibration became aven worse. I have recantly changed the rear end fluid, also i had the trans rebuilt but the noise was there long before the tranny needed rebuilt. Im stummped I doont know if its u-joints, maybe the eniter drive shaft, or theres a problem in the rear end, any help would be greatly appreciated. Thank you!