Sounds like something is off balance

Hello,

So I would like to say this issue only started this year. But the car is a 68 Dart with the slant six, A904 and the 7 1/4 rear end. the rear end is out of some early 70's abody from a junk yard that I put about 25000 miles on. The outer axle bearings were replaced by me before the car hit the road initially.

What I am experiencing is a noise that sounds like something is off balance. The frequency of the noise changes with speed, but the volume stays consistent. When driving it seems like it was coming from the right rear, so I switch the rear wheels but the noise did not change location. I also greased the U-joints when I did this. I thought that greasing them did quiet the noise but I could have been mistaken. It has about 20 miles on since that tire swap and U-joint grease, and the noise is still present.

Could this be a U-joint issue? I disconnected the drive shaft and inspected the U-joint caps to make sure they spun freely in the drive shaft and they did.

My concern is the 7 1/4 is about to have a catastrophic failure. In a couple weeks I plan on taking the car down to OBX from Pittsburgh (approx. 1000 mile round trip) and would really not want to have this rear end blow 100s of miles from home. What am I dealing with here?

Thank you