The best way to know the ratio is to count teeth, but estimating the ratio can be done as you did, rotate the tire(axle) while counting the number of turns of the drive shaft. The only question I have is in your counting. Most ratios in rears for trucks a at leat 3 to 1, many 4 to one. 2 to one is unheard of with very few exeptions, those being 1980's or newer car rear ends. The lower numerically the ratio, the faster the highway speeds, ie 3 to 1 allows you to drive faster than 4 to one, with corresponding loss of torque multiplication. I dont know the ratios offered in your years, but most in that era were 4.11 to 1, some also 4.56 to one. I think on a half ton it might be possible to see as low as 3.73(factory) and now some aftermarket companies offer a 3.55 to one.
I suggest a slow recount, make a mark so you dont get confused.
