Some years ago with a T1R I raced on an indoor wooden oval (cycle track). I do remember putting stiffer springs on the outside wheels to good effect on the end bankings.
Re diff. Yes problem
Actually not a problem but that is just the way they go when they wear - you need to fix it. It probably depends on costs but generally I found you buy a new diff for almost the same price as replacing parts. If you want to try repalacing theparts
You will need new diff balls and diff washers/plates. You can either buy separately or as a complete set. Not sure if your ball diff is the same as that used on later models (strongly suspect it is) but I am sure one of the XRAY guys can help you with that and part numbers. You will probably need to choose between steel balls or more expensive carbide balls.
You may also need a new diff thrust washer kit. If so I suggest you get a pack of cone washers too. (you will need a small guage circlip remover too)
In the UK the cost of the above parts using steel balls is 29GBP compared with a new diff for 40GBP. Bear in mind that a new diff comes with new composite output shafts, which if bought separately cost 24GBP for the pair - and yours may well be worn too.
HTH