YJ's didn't have a decal on the door jamb like most other vehicles have.
The normal gear ratios are...
6cyl 5spd...3.07, 4cyl 5spd 4.10, the autos are where it gets tricky, some were 3.53, and some were 3.73, but those are the norms. anything else you would have to pull a diff cover and read the numbers on the ring gear. Most of the outer diff tags have gotten torn off over the years, or a PO switched the gears.
ALL gear ratios are stamped into the edge of the ring gear, that is the only way to be 100% sure of what the gears are.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...