you can't see the collar. Where the angle gear bolts to the transmission the Collar connects the output shaft of the transmission to the shaft of the bevel gear. Both are splined. It is basically a splined coupler. Any leak would be unrelated.'

The collar itself is about $100. I saw in a post somewhere on here that the labor to remove and reinstall is around 4hrs but I can't be sure.

Not sure how handy you are but it is not a tough job to take out, just requires some patience...