I don't know why it would have gone into 4lo without moving the switch but to get it out you will need to be in neutral. Also, if the the service 4wd message comes up on the DIC it wont switch out of 4lo (or any other position). I had mine stuck in 4lo this winter and had to do the following:
1. With the truck off, leave/move the switch in the 4lo position
2. Turn the key on (no need to start, just key in on position)
3. If the service 4wd message pops up, turn the truck off and wait a few mins and then turn the key on again.
4. Once/if the service 4wd message is not lit when the key is turned on, shift the truck into nuetral.
5. Turn the switch to 2wd
6. Ideally you would heard the encoder motor un-lock the transfer case and the front diff actuator un-locking the front diff.
If step 6 didn't happen, I would repeat these steps several times. If repeated attempts fail, you may have some serious problems, that I really can't help you with. Even if you do manage to get it out of 4lo you still might have problems that need addressing, as I am. Currently I'm battling a service 4wd message and some odd noises from the front end.