It's the difference between a 31 spline axle (pre-99 Frontiers, older Pathy's and Hardbodies) and the 33 spliners that we all have (post-99 Frontiers and all X's).

You can do this though. Swap out the axles from an early Frontier, with your non-LSD 3rd, and then you can use the Lock-Rite or the Detroit. But you won't have your ABS any more, as they didn't have the ABS rings on the early shafts. Also, the early shafts have a 0.005" difference in size, being that they are smaller.

One member here did that, with great success, I believe it's OnlyoneDR.