Originally posted by sponger
"What else is there that wears out that could majorly affect the handling of the car?"
Its your diff 
I love the Pro 4
My car would push getting into the corner then the rear end would want to step out on me coming out of the corner (FTTC3 never did that). I tried everything to get the car way I wanted it, I put in a one-way, messed with droop, sway bars, springs, shock oil, rollcenters, camber link lenght, put associated parts in the diff, and even tried different bodies. 
All the changes I made were helping, but the car still wasn't where I wanted it ...setup wise. Then I came to find out my rear diff was too tight. A buddy of mine took my diff apart and found that the diff grear was causing the center bearing to bind (the bearing was fine). He put a diff together for me before my main the car worked just way I wanted it to. The car had a ton of steering in and out of the corner, I could get hard on the gas coming out of the corner and the rear end didn't step out on me once. Point is every car as a weak point, the Pro 4's just happens to be the diff.
Thanks man, my friend had the same problem and he said he fixed the inconsistent diff action by gluing the plastic ring to the gear, maybe your friend positioned that ring better. I did glue to ring on but it was very slightly not straight. Ill try gluing the ring straight onto an old version gear and see how it goes.