That rear bush is removable, you just tap it off with a soft mallet, then put the new one on, I usually use a large socket & hit that to drive the bush home. Check to see the back half of the housing isn't cracked. I can still remember Woody blowing this part of the gearbox off at speed. He was chasing leaks from the back seal for a while, maybe thats the problem. Good spray of some degreaser & a clean up should show up any cracks.