Hello
First look at the backing plates where they mount to the axle tube, look for warping and bending, see if it looks like the backing plate has tried to twist around with the brakes, also be sure that the mounting for the plates are tight if they have not twisted, if they have twisted the plates and all brake hardware should be replaced
If the backing plates are good then what has most likely happened is that the brake drums and shoes have developed hard spots, these can not be machined out, what happens is that part of the drum has become harder then the rest of the drum so that as the drum heats and cools, the hard spot heats and cools at a different rate then the rest of the drum, causing the drum to act like an egg and give you the thump,
You will need to replace the drums and shoes, at the same time inspect the wheel cylinders to be sure that they are not binding up
What happens is that something hangs up and causes slight to severe overheat of the shoes ( wheel cylinders, e-brake cables, driven with e-brake on ) then when you stop the brake shoes localize the extra heat and the drum cools unevenly causing a hard spot
The other thing in the back of the TJ that tends to make noise that sounds like a thump-thump-thump noise is the spider gears inside of the differential, but when these are starting to go bad or chipped they normally also make noise on acceleration and on cost at steady speed
I hope that this helps you
If you need more help or information just let me know
Thank you
Donny
