I have a 99 Suburban 4WD, 156k, and I am having problems with the 4x4.

Here is what I know.
1) the Service 4X4 light is NOT on, nor has it ever come on.
2) when in 2WD the light on the switch goes on and off
3) it will not go into 4WD HI
4) I can not get it to go into 4WD Auto
5) I can get it to go into 4WD LOW and it makes a thunk
6) if I move a couple of feet in 4WD LOW it pops like crazy.

Where to start?
Thanks.

 

I had to replace the selector switch in my 99. I had a lot of the same symptoms but not all.