That's it. I installed the slotted/drilled rotors.
No, that’s not it. Slotted rotors are a waste on a TJ, but that alone will not cause your lock up problem. Something else is going on - a few options have been touched on above.
Slotted rotors are designed to shed gassing when the brakes get hot - such as in a track situation - to help reduce brake fade. They do not improve braking under normal circumstances, and because of the reduced surface area compared to a similar sized standard rotor, they
could actually reduce performance in normal circumstances.
The other downside as I understand it, but have never personally confirmed, is that off road the slots and holes can accumulate dirt and debris.
My money is on improperly adjusted rear drums.