My friend with the flies in his teeth is probably right then. If you only have a third of the spline width left, that could be your problem.
Weld on replacements can be had, but I would look for a used final drive. R65 ones are usually available as most people change the fd. For a higher ratio with engine mods.
If you have a variable speed gun drill, they are very easy to test just sat on a bench.couple up with tape to the input, spin it at a slow speed and listen. I have a vid on YouTube.