1999 GMC 2500 pickup w/ pushbutton 4wd. The transfer case seems to be stuck in 4wd(you can't turn the front driveshaft by hand when it is in 2wd, even with the CV shafts removed) It made some noise with the shafts installed when running, and we took out the cv shafts and it doesn't make any noise. We can hear the motor on the transfer case work, and the dash lights for the 4wd seem to work OK. The "Service 4wd light" comes on and goes out on it's own. Had it on a OBD tester and it didn't show any codes. Any suggestions on what's going on?