If your ranger has the 8.8" rear ring gear your in luck. Several companys make Limited slip. I have the stock limited slip in the back of my 2002 4x4 offroad. Works awesome. My buddy keeter just ordered a Powertrax No-slip traction system for his 04 ranger 4x4. This awesome unit combines the the power of harsh lockers, with the qiet smoothness of lower traction limited slip posi. The guy he bought it from had a video of a 1999 2wd 1500 Ram that was playing in a snowy ditch that had the unit installed. He could go almost any where the 4x4s could. Keeter liked what he saw and ordered one up. $500 is what he paid. I'm sure if you shop around you may be able to get a better price.
1999 Ford Ranger XLT ext. cab 4x4. Level 8 Tracker wheels. Nexen Roadian MT LT235/75R15. K&N air filter. Sony MEX-BT3000P Stereo. Rockford Fosgate Power T1682 6x8" speakers.
2002 Ford Ranger EDGE 2wd. Thrush welded muffler. Spectre air filter. STK Pro Rack. Curt Hitch.