was the converter taken out while the motor was out and the front seal replaced, and didnt reinstal the conveter far enough. i have run into that too many times with the backyard garages and the somehow they find a way to bolt the thing back up other than that check the fluid level and condition and the linkage could be messed up so it will shift directly from neutral to 1st and pass drive and 2nd, have seen a couple of those. Thats about all i can see if no one took the pan off Hopes this helps.