2002 GMC Sierra 1500 4WD Trouble!!!

I have a 2002 GMC Sierra 1500 extended cab truck with 145000 miles. The truck runs great. The only problem is the 4WD will not engage. When I try to put it in 4hi,lo or auto the light just flashes on which ever one i chose. I can move the truck forward and reverser and nothing helps. I can hear the auctuator working and the gear ratio will change. If i go to for low, the truck drives like it is in 4 lo. The light next to 4lo continues to flash and will eventually go back to 2wd by it self. I have checked all wiring and fueses and everything seems to be ok. I have had no trouble codes and the service 4wd light has not come on. The only work that i have had done to the truck is a new transmisson applied. That was about a year ago and only 15000 miles. The 4wd worked fine after the transmisson was replaced though. I don't take it off road much, but it is about to get hunting season and you never know when you will need it. If anyone can give me some pointers, or if there is something i am missing, please help!!

Are you sure the auctuator is working? it uses a motor to heat up a gas. this gas expands causing the front diff to engage. when you say the truck drive like its in 4low thats because the transfercase is on 4low causing the rear wheels to be in 4low. that narrows it down to the front diff.

try draining the diff and seeing if there is any metal shavings, well the diff is empty try pulling the auctuator and see if it is working. if there are metal shavings then u got a bigger problem.

if the auctuator is not working i would suggest replacing it will a cable. they work great, i have one in my truck.