You can rebuild the original but it's not easy finding the right parts and kind of a pain. For about $200 you can get a brand new unit, fully greaseable and ready to go. A simple bolt in. The rear shaft isn't the problem the front shaft is, though it might pay to inspect the coupler between the tranny and shaft. It wears out and is also very easy and cheap to replace. It's that front one that worries me. It's a bad design (one of many on this model) and it should be addressed before you drive it again. Catastrophic failure awaits! Sorry, not trying to be overly dramatic, but it will leave you stranded at the very best and, more likely, without a transmission.
2004 Disco II, bone stock, daily driver (gasp!)