My service 4wd light coming on and off after the last snow. If you put it in auto it wants to go into 4h so we leave it in 2wd. Tonight I messed with as I finally had time. I manually switch to 4H, drove fine straight but had a whine, the when i turned the wheels in 4h it locked up and wouldn't move. It will not lock into 4L at all manually and takes a long time to switch to 4H from 2WD as well. Not sure where to start.

 

my light came on, i screwed off fill plug on front transfer case, stuck finger in there to check fluids and had zero feel of fluid. filled to brim and "service 4wd light" gone and switching to 4wd works again.. wouldn't hurt doing same for rear diffrential..

 

The fluid level is the cheapest place to start. There are a lot of threads here regarding the "Service 4WD" message. I learned that reading the actual trouble code is well worth your time. On my 2003 it was a bad sensor, reprogramming the T-case module, and cleaning up the ground connections at the frame on the drivers side under the drivers door. Good luck.