What exactly is a rear differential? Is that part of the gearing like a chain and sprocket would normally do?
If so is it possible to put a higher geared rear differential on my 1986 fourtrax 250?

 

All but a handful of quads, have a solid rear axle, and only a very few have a differential back there.
But i know what youre getting at, and yes the gearing can be changed..if you can find someone who makes an aftermarket ring or pinion for it.
Heres a site with info on differentials, and how they work.
how things work