XRAY - Model racing cars

I had the same issues with a newly acquired T1FK05 today. Spent all day at the carpet track [rubber tires] trying to sort out a setup.

Here's the changes I made, in order, that got the rear to behave:

Set rear toe-in to 2 deg.
Added 1mm rear droop [between 0 and -1 downstop]
Increased rear camber to -2.5 deg.

I started out the day with a car that drifted very well, and ended the day with a lot of grip at each corner, but now have to sort out high speed grip rolling.

Try changing to a shorter and/or lower roll centre, you usually do the same for the front and rear. That will make the car "dig in" more on corners with more camber roll.

Also put a shim (or a bigger one!) on the rear hub underneath the outer camber link...

Also try loosening the diff, although you did mention that already...

Run 3 degrees toe in on the rear and use a setup station to make sure it is actually 3 degrees, one side of my car is slightly less so I have to shim it out with a 0.2 shim...

More camber won't always work but it's always something to try so about 2 - 2.5 should be good...

Also! does the problem get worse throughout the race? if that's the case then the diff may be overheating... Try rebuilding with Schumacher diff lube and see how you get on...

Loads of stuff there for you to try!