Rear Tires Suddenly Lock Up Going 65

Well this was an interesting experience.. I pull the duster out onto a rural highway, manually shifting, 1-2, 2-3. Going about 65mph and I feel something funny, like a lack of power, I think it's a fuel problem, maybe I bumped the fuel pump switch by accident, nope. I hear sqealing tires and the rear tires (both) are compeletly locked up. After fighting with it from going into the ditch I come to a stop with a ton of tire smoke and huge skid marks behind me. WTF just happened I thought to myself. Engine is still alive. Brakes are good. I shift through every gear and they're working as advertised. I then drive to work normally, no issues. Hopefully you guys can shed some light onto this.

Specs and what I've done recently since the past 5 or so cruises:

73 Duster, 360, 904 tranny, 3000 stall, 3.55 gears w/ suregrip

Just changed the tranny fluid and filter and adjusted bands (72 inch lbs, back off 2 1/2 turns). I did find some orange rtv in the pan, 2 small chunks. Also drained the converter and refilled with dexron atf.

Added a little brake fluid, peddle was a little soft. Good now.

Topped off the radiator.

I'm guessing a major transmission malfunction, any ideas?