The XX gearbox is essentially the same as what was used for the XXX series of buggies - & even in mid-motor Atomic Carbon CR2 guise these were quite happy to handle the brushless motors we were throwing at them - albeit with the more regular maintenance associated with running a ball-diff over a geared diff.
For the record I have been running a 6.5BL (an early 'mild' Hacker motor controlled with a blinky-spec LRP Sphere ESC) in my XX KW without any issues.
Its certainly worth re-building your diff/gearbox - give a good clean - re-hone the diff plates using wet & dry paper on a flat board & re-build using black grease in the thrust (AE or equivalent) & white silicone grease on the diff balls themselves. Likewise with the shocks - you can't go too far wrong with the kit set-up - red kit pistons & fresh new 30wt oil front & rear - green springs on the front & reds or pinks on the rear.... that said, I'm not actually sure what's in the shocks on my car - like you I first tried the car with the 20-odd year oil in the shocks & it went OK... to this day I still haven't re-built the shocks & the car was good enough to qualify for the
Race of Champions at the first Revival & won the A-Final in its class/age category this year!
