BEST way - take of the cover, count pinion teeth & ring gear teeth, divide ring gear by pinion.
Second best way - jack up either end, turn the driveshaft until the wheels go around EXACTLY once, and count driveshaft turns.
Alternate method - check for an ID tag retained by the cover bolts. It will have the Dana BoM number, a few other numbers, and a ratio on it. It will also tell you if you have Trak-Lok, and need to use LSD "Friction Modifier" for refills.
If the tag is missing, check the other end. I can't give you a fixed location for the rear tag, but the front will usually be on the passenger side of the cover (on the straight.) Both axles will be within a couple points of each other - otherwise, the drivetrain would grenade in 4WD.
Note that the tag does not apply if the gears hve been changed - and counting teeth requires removing the cover.
Typical ratios for 4.0/AW4 are 3.54/3.55, 4.0/MT 3.07/3.08, and four-cylinders seems to end up with 3.55, 3.73, or 4.10 gears. Early trailer tow packages seem to like 4.10 gears - but that's only up to 87-88-ish.
Good luck!
5-90