Triumph Repair: rear axle replacement, rear axle shafts, triumph spitfire

Question
  I think the knocking sound is coming from the rear axle possible worn splins,what is a good way to fix this  and  is there a replacement axle from another car company I can modify to fit   steve johnson

Answer
I assume you are speaking of a triumph spitfire, as you don't state the make of triumph.  Usually the knocking sound comes from a bad u-joint, not bad splines.

You can check by putting the car up on jack stands, and with the transmission in gear and the parking brake on, attempt to move by hand the driver shaft, and two rear axles.  You should find that one of them moves more than others (they shouldn't move much, if at all).

There are several different options people have tried to replace the rear axle shafts; there is no one easy swap, and they all require modifications.  The easier fix is to replace the u-joint.  If the splines are indeed bad, then a swap of a used axle shaft & hub/housing unit is fairly easy to do.