A tight rear diff and a loose front diff might be a drifting setup.
For grip racing you will run a free rear diff and a tight front diff (or even a spool in the front).
Refer to the instruction manual for information on rebuilding them, but a few general tips for ball diffs are...
- Clean the parts thoroughly
- Use plenty of black grease on the thrust bearing and a smear of silicone ball-diff grease on the main balls
- Never over-tighten the screw. Initial setting should be hand tight and then backed off 1/4 turn
- Adjust for feel in the car, 1/8th of a turn at a time. The right starting point is a free movement but not free spinning
- The diff should not slip in use either. To test, hold one wheel and the spur gear, and try and turn the opposite wheel. It should take a good amount of force before it slips
There is an art to building ball diffs that only experience can provide. So have a go, it might take a few builds to get them right, but it is worth it.
The new oil-filled gear diffs are easier to set up though.