On my 700, there's a small metal tag on the rear of the axle, on the driver's side. It was covered with crud and I used brake cleaner to clean it off.... it also took off all the markings >.<

There may be a tag on the axle somewhere. It will be flush if it's there. Might as well look for it.

Good Luck.

PS I think you can also just measure the rotations of the driveshaft vs. a rear wheel.

not sure if the was a tag, or if it is still there, but the only fullproof way of deternining the ratio, is to pull off the cover (you are going to want to do this anyway to see if there is corrosion on the differential components) mark the ring gear (big one) and rotate it all the way around while counting each tooth; do the same for the piniion gear (its trickier) I usually put a dot of paint on the end of a screwdriver and touch it to one of the teeth then rotate it . It may take a couple of times around to be sure of the count. Divide the large number by the small one and you have your results.
poorgie

Someone else will have to say if it's a tag (would be attached to one of the diff cover bolts)--or a glued on white printed tag (on the cover or one axle tube)--I just can't recall---but I can tell you a simple way of checking the ratio. Mark the input (pinion) flange and a reference point on the diff housing--- and mark one rotor (or wheel flange) and a reference point. Be sure the other axle is stationary and then rotate the marked axle two complete revolutions while counting the revolutions of the pinion shaft. The number will be the ratio. Depending on the year/model of the donor the ratio could be 3.91, 3.73, 3.54, or 3.31 . If you should happen on a diff that won't spin freely while the opposite wheel is locked up you may have stumbled on an LSD rear. Good luck---Dave