Something is wrong with the 4WD. Will engage in neutral to 4WD Lo which means the vaccum is probably working but 4WD Hi will not engage. I have tried resetting the control module as described in the owners manual. The switch used to shift works fine. i am thinking it is the accuator, the transfer case motor or the module itself. let me know anything if you can.

 

my 99 jimmy does the same thing.heres what i have found out . the problem is in the harness that plugs into the 4X4 control module.apparently water gets in somehow and corrodes some pins in the plug causing your problem.module is located under kick panel on passengers side footwell. hope this helps