Does anyone know where I can get a rear end locker for my 2001 tracker

 

First you need to identify your axle type and size. Then there are many places you can try.

The junkyard, 4x4 outfitters, rebuilders, differential dealers, countless online retailers (ex Rockland Standard Gear Inc.), your neighborhood parts dealer ...
Even Autozone carries locking differentials.

 

Start here: https://www.rocky-road.com/kickarb.html