quick and dirty way to tell gear ratio:
put truck in neutral
jack up one rear tire
scratch a reference mark on the rear driveshaft
rotate the tire around exactly twice while counting the revolutions of the driveshaft
that count is your gear ratio (3 and a half times = 3.55, just over 4 times is 4.10, etc)
if the axle has a posi, you'll need to jack up both tires and spin one tire just once (meaning the other tire will also spin around once at the same time).