I had the same problem with my '05. I went in to the dealer to ask them about it and apparently it's pretty common for them to seize up. They put it on the lift and gave it a try but they couldn't get it down either. The offered to cut up the mechanism and sell me a new one, go figure! I ended up taking my bed off to install drop shackles and I was able to free up the latch and get the tire down then. There is a secondary latch that I had to beat with a hammer and punch to get it to release, based on my experience I don't think it's possible to release from under the truck.
This is the only pic I have of it, the latch is inside the cavity behind the rubber boot.

This is the only pic I have of it, the latch is inside the cavity behind the rubber boot.