Try to torque the pinion cup nut to the right specification. I refurbished
a couple of final drives. On one of them I had gotten a friend of mine
to install a pinion cup at a previous time. He asked how tight. I didn't know.
I said "plenty tight". That was too much! I couldn't remove the nut later. I
had to carry the final drive over to the NAPA machine shop guy, his giant
compressor and good tools got it off.
It is probably hard to tighten the nut to the right torque. It seems to me
(I don't know anything about fancy torque wrenches) you need to defeat
several degrees of freedom within the final drive in order to stop the
shaft you're trying to tighten the nut on from spinning. Or the whole
final drive from trying to spin.
I built a rube goldberg machine:

The machine exploded the time I tried to use it to remove the too-tight
nut.
Over at Stanley Steamer's we just fiddled around with some clamps while
positioning his final drive on some steps. I think we actually screwed
a block into the step to keep the final from trying to spin around.
-Mike