[blazee]

I doubt that it needs a complete rebuild. P1870 means it has detected that a transmission componet is slipping. When the code is set, the fluid is automatically boosted to maximum pressure, this is what is causing the hard shifts. The most common cause of P1870 is the valve body, these transmissions are known for having sticky valves in the valve body. I have a service bulletin about it that I can email to you if you want it.