1976 C10 - Trouble moving - Automatic transmission

[kcackler]

I have a 76 C10 that has been sitting for about 8 months due to being busy with life. Now that it's nice again, I'd like to start driving it some more, but I can't get it to move. I can start it and put it into gear no problem, but when I put on the gas, all it does is rev up. Once I get it to maybe 4-5K RPMs (No tachometer, just guessing), one of the rear wheels finally starts to spin, but the other one remains stuck and I go nowhere. I put it into neutral and was able to pull it into a new parking spot and both wheels turned while pulling.
I'm not sure where to begin troubleshooting.

Any advice would be appreciated. This is a project truck and it's entirely likely that something is seized, but with both wheels turning when I towed it, that seems less likely.

When it was parked, it drove beautifully. We're moving in a month, and I'd really like to drive it to the new place rather than tow it.

I'd like to start troubleshooting this weekend, so any tips / troubleshooting steps would be appreciated.