could very well be the clutch. when you shift and it grinds will it actually go in gear or it won't go in gear at all? If it grinds when going in gear but actually does gear its probably the trans if it won't go in gear at all but the transmission bench shifts ok its probably the clutch. if neither of those are right remove the driveshaft, cap the output shaft hole, start the car and see if it shifts, if it does chances are its a driveshaft issue.