Used to do that in my 1/10th scale Traxxas Stampede. It is definately a tried and proven method for tuning gear diffs.
Try different thicknesses of grease.
I've found 'Green Slime' works great. Most LHSs with RCs carry it.
I just wanted to add that while this slows the differential effect, it is not 'true' limited slip. A true limited slip ball diff actually allows little bit of slip, this prevents torque from breaking traction.