TCM is shot. Mine would do the same thing...it's also temperature dependent; tried to move it one day when it was warm and it wouldn't do anything...tried again a few days later when it was about 20° cooler and was able to drive it around the yard.
My truck's been sitting in the yard collecting leaves, pollen, and bird ****** since November. If you're willing to spend upwards of $7500, you can find them on Ebay and several online performance stores. I sent mine off to Xemodex a couple weeks back and just got it back today (the 2 retainers holding it to the fan shroud broke pulling them out so I've got to find a couple of new ones tomorrow). They repair them for $1200 and reflash to your VIN...found them recommended on durmaxforum by several people. Hated to spend the $$$ on a repair, but it comes with a lifetime warranty. Spoke with 2 GM dealers here in town and they said they haven't heard anything about when new ones would be available.