have you checked the relay for the t-case? could be you motor that locks it into 4wd. there has been several post here recently that has the same problem. just search for them and there was all you details about it.
I would try 2 things. First have someone sit in the truck and you try to manually move the actuator on the differential while they turn off the 4wd. It should move out towards the driver's side when the 4wd
switch is turned off. If that doesn't work, put the truck in 4low and then turn the 4wd off and put the truck in reverse. If that doesn't work you probably have a bad motor on the transfer case, if it's getting power.
It's hard to tell without more info. Are the 4wd lights turning off but
truck remains in 4wd. That could be a solenoid or a vacuum actuator