Re: Changing out fluids
If you are in a warm climate, you should be ok with 80w-90 gear oil, 75w-90 will allow easier cold shifting though. I would recommend against ATF, except if you live where it freezes in winter. The original factory fill was a weird smelling 80w red/brown oil, that the t-case also used. There was a recall in '82 on '80-'81 CJs with T4s and T5s to replace the ATF with gear oil.
Hope this helps, good luck!