Bad universal joints on the drive shaft? Sheared key? How much of a slip are you getting?
Where do you drain and add fluid ??? I have the same problem.Have a 318 that jumps in and outta gear in forward but not reverse. I did change the fluid and still have the same problem. When going uphill it's also fine and seems like if there's a load on it (like uphill) it don't slip. But on the level it's bad. Again,,,,reverse is fine.
You drain the fluid underneath the rear axle. Very simple and takes about 10 minutes. I also flush mine when I change it out.Where do you drain and add fluid ??? I have the same problem.