Sounds like the L/R band. In a stock trans the L/R band is not applied in drive. Only in manual low and reverse. You going to have to take it apart to replace it. Plus your going to have to do alot of cleaning. You might also have to replace L/R drum. Do you know if they left the over running clutch in? If they removed it the car will free wheel in manual low when you let off the gas. Sounds like it still in it.
Good Luck.