Cool. What would I plug the axle tube with? And from your earlier post, would I be correct in assuming the vent hole is just capped in the boss cover rather than me having to drill the hole out?
No drilling required. Just remove the vent from the axle tube and install it in the top of the cover.
Plugging the old vent hole in the axle is kind of up to you. It is not pipe threaded so I just took the vent with me to a parts store and found a brass fitting with the same thread on one end and a female 1/8" pipe thread. I put thread sealer on the fitting and screwed into the axle tube then put thread sealer on the 1/8" pipe plug and screwed it into the fitting. That's all I was able to come up with, you might find something different that works for you.