Secure the truck so it can't roll away on you, put the trans in neutral and jack up one of the rear tires. Make a mark on the driveshaft that you can see so you can count it's revolutions and then watch it carefully as you spin the one tire 2 complete revolutions. the number of times the d-shaft spins will tell you the ratio. Just over 4 times would be 4.10, just over 3 times would be 3.07, etc. If it turns out you have a posi in the rear axle, you'll need to jack up both rear tires and do the same thing, but this time only spin the tires around once.
Anyone can feel free to correct me if my brain got that backwards at all. :D