General questions from a new owner

Thanks for the quick responses, guys! GTS, I'm kinda partial to my rallye wheels and I'd rather not change them. When I change the rear end to an 8 3/4, will that fix the issue? Do those axles have a wider track to them? I didn't think to check for cracked leafs. From a quick glance, they just look worn out. I didn't consider air shocks. It has some old-ish Monroe gas shocks all around and I plan to change them out, as well. Will new shocks alone help? Or will the lack of spring in the leafs just compress it?

As for the oil...it's not leaking anywhere else. The engine is only a few thousand miles since a total rebuild and I just went all through it and tightened all the bolts from the usual suspects and cleaned it up. It's all still clean after a few hundred miles, and the only oil that's leaking is pooling in the bottom of the bell housing. I still say it's the rear main, but I thought I'd ask if there is anything else back there that was just slipping my mind.

Memike, I think you're right on about the u-joints. I'll just change them both out and call it a day.

Another question I forgot to ask: My 904 is a three-speed. I want to say my old '83 Ram from back in the day had a four-speed. I'm I crazy? If it DID have a four-speed, can I convert mine, or do I just need to replace it? Thanks again!