Just to close out this thread as an FYI. The motors that I bought from amazon are ALMOST an exact match. Three exceptions: 1. The flat spot on the shaft that the wedge seats in was longer than the 4th gen but had zero impact on use. 2. The polarity is reversed but easily fixed. (the running board wants to go in when the door is open and out when closed) To reverse the polarity, I simply cut and swapped the wires on the side of the harness plug with two pins. When plugged into the vehicle, the motor responded correctly producing the 1/4 turn in the correct direction (clockwise/counter clockwise) to operate the running board. 3. The holes that the plastic cover is screwed into are not threaded and may be drilled slightly bigger than the original part. I just tapped it and used a bolt out of my coffee-can-of-bolts.
After all that, I got the same issue, starts to retract but comes back out. Finally broke out the oil pump can and liberally applied regular motor oil all over the hinges. Yes this fixed it. Now every time that I change the oil, I drench the hinges on the running boards. I left the new motor on.