I had an 08 that had random 4wd problems. Mine had the auto 4wd option and if you use that option it only worked once then quit. It also didn't consistently switch to 4wd or 4low and would just flash the 4wd light. I took it to the dealer and replaced the t-case shift motor and that worked fine for a short time. I took it back and they warrantied that motor and the same problem, worked fine for a short while then killed that motor. They ended up replacing the shift motor again under warranty and changed out the TIPM and the problem was fixed. Of course it snowed every single time I took it into the dealer, and it rarely snows here lol.

Have you checked the make sure it is not a mechanical failure?