I would start at the simplest which is check all of the electrical connections including grounds for the solenoids involved in the TC lockup. Sounds like you might just have an intermittent connection. Other than that some debugging could be done by checking voltages etc.
Some related info...
When you put the new ones in you will be pressing them in towards the outside from the diff side. Make sure your socket is bigger than the inner lip of the seal that has the spring. I recommend removing the spring during installation just so you don't damage it. also put some permatex or...
Same situation here, just got an XJ this week, once in a while got a huge vibration, finally tracked it down to happen when the electric fan came on. found a crack in the fan, have ordered new one.