Front drive shafts worn, noisy and getting replaced.  About 120,000 kilometers.  Mechanic tells me centre differential getting a bit loose and some noise.  Is there anything to fend off the high cost of replacing this part?  Back off 50 years and we used to add molybdenum or products like bardol.  Thanks for your help.

Welcome - problem is the additives you mention were to prolong the life of parts prior to wear, more of a preventative maintenance approach when oils were not as good / resistance to shear etc in high load mechanical situations

You may well hide the noise for a bit but the wear already will mean tolerances are now looser and will continue to get worse