Welcome to AF!
Wow!!
If all that work was done to fix this problem, thats a lot of bux and not have it fixed yet......
With you description of the problem going away when the ABS lite was on, I would agree that it is likely an ABS system related malfunction. When the lite is on, the ABS is disabled in a "failsafe" mode.
Is the "feel" of the problem in the brake pedal and transfered to your foot? Have you (they) ever driven the jeep on ice and applied the brakes? Is the feel similar?
First thing to check with an ABS problem is the tire make, model, size and pressure. They should be identical on all four corners. The tread wear should also be as close as possible.
Check to see if the ABS set any codes when the lite came on.
Might it be that the actual problem has changed slightly since the work was started and we are now dealing with something different but similar?? With the range of adjectives that you use to describe the symptoms, that seems a possibility to me. Hard to know at this distance. Not doubting you here, just asking to give a new perspective.
A sensor could have been damaged after the original problem was fixed or when some of the parts were replaced. The sensor should be check-able by someone who knows how to use a multimeter or with a scope.
JD