I'd always rebuild the diff before a big race meeting but this is what I do for club racing
Ball diff as above. If it feel notchy clean and grease it, if it feel smooth leave it.
Gear diff, I've never used one in a buggy. But if there's no signs of leaks I would leave it alone. I'd recommend replacing the seal between the two parts of the housing when you rebuild it, they only seam to seal once in my experience with touring car diffs
As for wings, I run the rudebits R1 wing. It's virtually indestructible, which is handy with my driving.
