An '85 should have an AMC Corporate axle in the rear (the M23). Look here for the axle codes: http://www.jeeptech.com/axle/amc20.html
The front axle should have a tag, as mentioned. Likely it's 3.31 or 2.73, by 1985.
Re "the best" gearing, for off-pavement the higher the number the better. But you may have to compromise to get acceptable engine speed on the highway. Decide what engine speed and highway speed you want, then use one of the online calculators to find an acceptable gear ratio. http://www.novak-adapt.com/knowledge/gearing.htm
- for example, 35s at 65 mph and 2500 RPM is about a 4.10 axle ratio.
1) '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps. 2) '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR 3) '75 CJ-6, 304/T15, PS, BFG KM2s, soft top 4) '95 VW Golf Sport 2000cc 2D