This is a very common issue with the 4th gen design. With lack of use the mechanism gets "gummed" up and will get stuck. Many people on the board subscribe to the process of putting the truck into the different modes, 4HI, Diff Lock and 4LO once a month to make sure all the actuators stay exercised.
There is a bunch of threads on here on solutions. Replacing, manually moving with force or electricity, etc.
I bought a 08 Limited V6 with a bad timing chain and had been sitting for quite some time. I had the same issue and followed the electricity thread and was able to get it back to functioning.
Fixed my T-Case actuator w/ electricity and logic.
Good luck, hopefully it doesn't cost you too much money to get it sorted. Replacing can be expensive as the part is not cheap $500-1000 and involves removing and splitting the transfer case.
Matt