17115 means the TCU has noticed a difference between the input and output speed sensors that does not match the expected value for the gear it thinks it is in. Mechanically, that's a clutch pack slipping, as you describe. Not low oil level - in that case the torque converter will start whining well before the clutches slip.
However, given that its been rebuilt, I'd agree with David that the F125 is the first place to look as that is cheaper and simpler to swap as a diagnostic measure than pulling the gearbox out. If that indicates a different gear electrically to what the gearbox is actually doing mechanically that can also trip this code. For example, the F125 briefly indicates 'D' to the TCU, with the shifter actually in 'R'. TCU goes 'whoa we're going backwards when we should be going forwards: wrong gear ratio!'.
On an older gearbox I'd be looking at the A clutch pack, as that's the prime problem area in these. Likely the roller bearing fallen apart, or the clutch basket cracked.
F125 first though as it causes a multitude of weird problems
