Ive got a 70 challenger 4 speed dana car and i drove it for the first time this year the other day when it got warm i went about three miles and it started jumping in the ass end like it had a knot on one of the tires so i changed tires and it done the same thing so i changed the u joints front and back and drove it and for about 3 miles it done fine and then it started jumping up and down again like maybe a axle bearing or something is getting hot and then starts to hop up and down in the ass end has anybody had this problem or have any suggestions any info is greatly appreciated thanks.

Pull the cover now(drain it first if it has a drain plug ) and see if there are any loose pieces floating around in it Let us know what you find, is the roads you drive on straight or curved when it starts making the problem?

put the rear axle up on 2 stands & have a helper run it up in gear while you safely have a look underneath. first as said open the cover & see if anything looks wonky or any big debris & I'd have it on stands for that too both for easier access & so you can turn the wheels

The Dana60 and 8-3/4" have axle bearings that need to be greased with wheel bearing grease. If the grease has dried up, the axle bearings will wear out, and they are what holds the axle in the housing. Pull the axles out and inspect the bearings before you loose an axle. I had that happen on my 1973 Power Wagon many years ago when I was 16, and was suprised to see my axle and tire pass me after going around a corner.

That is about the only weak point I can think of in an OEM Dana 60...the tapered wheel bearings that must be greased.
I read the initial post and waited for someone to chime in with a theory that sounded plausible and now I have read it

Cab's and RapidRobert's inspection procedures probably would have found it, too.

Check the pinion nut and see if you can move the pinion back and forth as well.

Sheldon

Now you've done it

I do remember changing a complete Dana 60 in a Ford 3/4 ton in 1986. The pinion nut came loose and it spat parts of ring gear through the inspection cover when the perfectly aligned parts were "compromised", to say the least. To be honest the company truck was being abused by a construction crew. And once we slipped another "road tested" unit in place, rebuilt the brakes, etc.etc. all was good.