4+4

It sounds like something could be sticking in the front differential. Have you checked that? Also, have you checked for vacuum leaks? Is the linkage properly adjusted to the transfer case? If you have an electric shift 4wd, are you sure the actuator is getting power when you request 2wd? Ti may be your t-case module. That is located in the kick panel on the passenger side. Make sure to check any fuses and relays as well.