1980 jeep Cj5 questions?

A '80 should have either a dana 20 in the rear unless it has been replaced by a 44. The front most likely has a 30 unless it has been replaced by a 44. The ratio is most likely around a 3.73 or 4.10. But by changing the gears you will get better top end performance. If its the 373, then 410 should be fine. Call a 4 wheel drive shop and get their advice. Oh, by the way, the only 30 model for the rear is a 35 which was used in the Wranglers.