28 spline axles are stock on all Panthers up to 2004 except for the Town Car limo packages. 31 spline axles came standard in all 05+ Panthers. To upgrade to a 31 spline trac-lok, you would need to replace the trac-lok unit as well as a set of 31 spline axles.
You can find out your rearend combination by checking out the following vin decoder compliments of Liquid
The advantage of 31 splines is with more splines it can withstand more hp and heavier launches on the track