Knock / thump from rear at highway speeds

I have a 1998 Ford Explorer 4X4 two owner and only 63K miles. All service records including tranny flush. Tranny shifts fine, there are no “clunks” when shifting out of Park.
The only symptom is an occasional bump or thud ( maybe more like a jolt )coming from underneath at the rear at highway speeds. Sometimes it is more pronounced than others. I don’t believe RPMs change when this happens. Around town is fine and only at constant speeds above 55 which makes it difficult to recreate. I have checked for anything loose underneath ( incl spare tire ) and can’t find anything.

Any suggestions on this rear end bumping will be appreciated.

Rubber exhaust hangers? Donuts?

Cavell - it’s almost a jolt …so I am guessing it may be more severe than hangers. Thanks for the post

The noise might be coming from a dry/worn U-joint.

Raise the rear of the vehicle of the vehicle so both tires off the ground. Place the transmission in neutral. Grab one of the rear tires and rotate it back and forth while observing the U-joints. If either are worn you’ll see it.