Unique Reverse Problem

[CCFV1982]

I have a bit of a Frankenstein project going here: I was given a 1982 Club Car 36V Fairway Villager that had been sitting for at least six years. After a thorough cleaning and deciding (from reading through this forum) that a new Alltrax system was the way to go, I ordered a conversion kit from Carts Unlimited (new solenoid, HD F&R switch 400 amp Alltrax controller, high amp cables and an FN1 switch) and installed it with brand new batteries. Nothing. After some motor troubleshooting it was decided to find a new donor cart for the rear end (better to have the newer spline motor). I was able to put my hands on a 1997 cart, discarded by a local golf club. I swapped out the rear end (differential/motor, suspension) and now have a cart that performs very well in forward, but to go in reverse I have to give it a shove. The controller is working properly in both directions, but in reverse I get some noise from the differential like it's trying, but it won't propel in reverse without a push. Once I get going in reverse it seems to do fine. Unloaded (rear wheels off the ground) it makes the same sound and "grunts" a few times but will then spin in reverse. But on the ground it just can't get going. I have read through other posts on reversing issues but most revolve around a new F&R switch. Any thoughts on this problem?