727 issues and questions

Wow, now that's something you don't see everyday!

To your questions, I'll give you the elements in use for each gear that way you can determine which one(s) are slipping.

manual low - low reverse (rear) band and rear clutch
low (starting off in drive) - rear clutch and over running clutch (free wheels on coast)

2nd or kick down from drive - front band and rear clutch

drive - front clutch and rear clutch

reverse - front clutch and low reverse band

Armed with this information you can determine by when the trans slips what element(s) are in use at that point. For instance if your trans slips in 2nd but not low than the rear clutch is good and the front band is slipping. If it slips in all 3 forward speeds but not in reverse than you know the front clutch and rear band are ok, it's the rear clutches that are slipping.

BTW you did check the fluid level and condition right? Has it ever been serviced?