O.K., the 4x4 was not kicking in on my 98 Jimmy. A mechanic at the GM dealership where I work stated that it was probably the encoder motor. I just swapped the encoder motor with one from a salvage yard. It seemed to work fine at first, but now it is still doing the same thing:

4x4 Hi will not respond/engage from 2Hi (either from park or neutral). When you push the 4 Hi button, nothing happens, or it flashes and kicks back to 2 Hi. If you put the vehicle in neutral, allow the vehicle to roll and push 4 Lo, the 4x4 lights blink and the 4x4 kicks in, but bothe the 4 Hi and 4 lo buttons stay illuminated. Are they supposed to both stay illuminated?

According to the owners manual, if they do this then the 4x4 switch needs to be adjusted. How do I know if I have another bad encoder motor or a bad 4x4 switch? Can the 4x4 switch be adjusted? What else might the problem be? Any help would be greatly appreciated.