XRAY - Model racing cars

The nylon nut is pre-installed in the short half of the diff. This nut is shown separately in the exploded view of the diff (at least in the FK05 manual -- I'm still running that car) but does not have a separate part number, so it appears that XRay does not sell the nut separately. It must be quite rare for it to fail. I've never had your sort of failure in an XRay diff; this failure happens pretty regularly in other diffs I've used, like Losi and Tamiya.

First be absolutely sure your thrust bearing is not at fault. Many diff problems are thrust bearing problems.

If I were in your position, first I'd take the diff apart and clean both the adjusting nut and the adjusting screw thoroughly with motor spray. Then I'd put a little CA on the threads of the adjusting screw, let it dry completely, and assemble the diff, being careful not to get any grease on the screw threads. You aren't trying to glue the screw to the nut, just to create a little more friction. If putting glue on the screw isn't enough you could put some on the threads of the nut, letting it dry completely, in addition.

Another traditional technique in situations like this is to take a bit of a plastic bag, cut in a circle, put it over the end of the screw like a little tent, and assemble. Same goal as the glue: take up some space and increase friction. The plastic bag trick might be difficult to pull off without having bits of the plastic bag going where you don't want and fouling up the diff action. Cut the plastic fairly small and glue it to the end of the screw; might work!

One of the techniques above will probably work for you. If they don't, as a last resort before buying a new diff, you could try to melt the nut a little with a heat gun. Or you could try getting a new nylon nut from the hardware store -- it might just press fit in there in place of the old one. Good luck!