if the rear end is fried you aren't driving it anywhere.
Either jack it up and rotate the wheels and count driveshaft rotations to figure out the gearing, or pull the differential out and pop the cover to check and count all the teeth on both the ring and the pinion to figure out the ratio.
Sometimes the wrong tag ends up on the wrong piece of equipment, or sometimes if a motorpool needs parts and they have a truck that's getting disposed of, they'll take whatever parts they need, and toss whatever the worst parts they have into the disappearing truck and call it good. Not their problem after that.
Without physically verifying what the gearing is, it could be anything.