Sounds awesome! Welcome to the group!
When I bought mine it ran the same (only when you pour fuel down it's throat).
You have a solid plan! I don't think the J20 axles will "bolt-in" but it should not be that hard to get them in. The steering is slightly different, too but not a big lift to swap that over.
Your truck should be a really fun project!
79 Cherokee Chief (passed CA smog check Sept 2020)
(Cherokee Build Thread)
11 Nissan Pathfinder Silver Edition 4x4
09 Mazdaspeed3 Grand Touring