99 SLT 4x4 not engaged

Hi all, I looked but couldn't find anything like this. My 1999 Jimmy will not go in 4x4 anysetting, it has auto 4x4 button, plus 2hi,4hi,4lo. They all seem to work right, light is lit on button pushed. You can here the front diff. ingage and the transfer case engage. When in snowed last week it was not in 4x4, in any range. I jacked it all 4 wheels off the ground ran it in 2hi back wheels turned then pushed 4hi front wheels turned like it was in gear OK. So I put the front end on the ground and it didn't move the truck off the jack stands. I need help on this one. :banghead:

Look it up in this link
http://www.4x4posi-lok.com/PDFs/TSG_chevyT.pdf

It's the best trouble shooting guide that I've seen

Nice link.

This may be of value also.

http://www.findarticles.com/p/articles/mi_qa3828/is_200406/ai_n9424000

This may be of value also.

http://www.findarticles.com/p/articles/mi_qa3828/is_200406/ai_n9424000

Thanks for the help. Got to test more on the truck today, we got snow, and found that tha 4x4 works with the 4x4 auto button, but will not work in 4hi or 4lo. Do you think this is a wireing or switch problem?