Why is my 700R4 completely frozen?

After sitting for a month or so, my 700R4 transmission is completely locked up. I start the engine, but when I put it in gear, it slams into gear violently and the engine dies regardless of the RPM. I jacked the rear wheels off the ground and it does the same thing without the wheels even slightly turning. I can rotate the wheels quite easily by hand while on jack stands.

Any thoughts on this greatly appreciated.