Hi Folks,
A high torque MUST be placed on the nut in order to keep the shaft under a tensile load.
If the torque is underdone it may allow flexing on the micro scale and cause failure.
I would suggest the tensile strength of the thread run-our area is far in excess of the load required to strip the threads in the nut.
I haven't a clue who manufactures your drive shafts but the 'repro' stub axles in a TR4A IRS as supplied by Moss are top quality. (I know yours are different).
Your first port of call should be the garage as they will know what has been fitted.
Roger