4WD automatic trans failure

1983 GMC sierra, 120k mi, 350 v8- had trans rebuilt 2 yrs ago, has worked fine. I lost all gears while cruising about 40 mph on level road. This was accompanied by a gnarley sound of grinding/spinning noises. I put the vehicle in 4w lo, and it moves, so I drove home very slowly(it was about 20 degrees out and my wife was with me, and we were not dressed for a 5 mi. Walk)is this an internal band/planetary problem? Fluid was not leaking or low.