If you still have the tag attached to the 3rd member housing you can decode it.
Or you could turn one of the rear wheels off the ground, if both rear tires start moving together you have a traction lock, if you spin one tire and the other moves in the opposite direction then you have a regular differential.
If you mean updating a 3rd member to a traction lock? if you have a ford 9" rear there are traction locks available from ford racing, you will need to know if your axle and yoke have 28 splines or 31 to order the correct one. The new ford traction locks will require a slight modification to the axle housing to work.
The kits they sell to convert a 3rd member are always missing parts like a pilot bearing clip or shims, a kit is a good start since complete bearings are provided but you will find your self ordering more parts after. Ratech is one of the only companies providing aftermarket support.
