unusual trans problem '81 380sl

[blu54eyes]

Car will not move from a stop when I put it in gear. I can feel it wanting to move when I feather the accellerator, then with a push (still feathering the throttle) it gets going and drives like nothing is wrong. It upshifts and downshifts nicely. But come to a stop and it starts all over again. It goes into reverse just fine.
All this started when I had to accelerate (from a dead stop) rather hard. There was a kinda thud and jolt like someone had gently hit the bumper but the car kept going - until I stopped at a light...
I was told I need a whole new/rebuilt trans, that the gears were gone.
What?! Then, how is it that once I get moving you wouldn't know anything was wrong? They had no answer.