You are talking about the splined steel slug that goes in the end of the axle? It's not generally a wear item. Are you sure it's assembled right? IIRC there is a spring behind it, then the slug goes in, then the snap ring on the end of the axle, and the dust cap.
I know you say it's not the U-joint, but have you looked at the AXLE u-joints or the driveshaft u-joints? That's a more common problem. Also look at the spring end bushings. The Quadratrac is well-known for making a into-reverse clunk due to worn splines on the transmission output shaft - another thing to rule out. Also rule out the double Cardan joint on the front driveshaft - a common failure that will take out your transmission case if it lets go.
If you think it's the slug that's the problem, you should be able to source another one from J&W in Antelope CA, or Jake & Jackie's in NC, or from the IFSJA parts forum.
1) '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps. 2) '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR 3) '75 CJ-6, 304/T15, PS, BFG KM2s, soft top 4) '95 VW Golf Sport 2000cc 2D