There are two ways of doing this. One, take off your rear diff cover. Count the number of teeth on the input pinion, and count the number of teeth on your ring gear. Divide the ring gear number by your pinion number, and that's your gear ratio. Not a practical way of doing it, but it will work.
Do it an easier way, just involves a little math. Put the truck in 3rd (1:1 gear ratio), and cruise at 60 mph. Note your engine rpm. Now, figure out how many revolutions your tires make in one mile. This is simple: take your tire height, multiply by pi (~3.14156), and then divide one mile by that value (watch your units, make sure you are consistent). Finally, divide your engine rpm by your tire rpm, and bam, there's your rear end ratio.
What year is your truck? GM is one of the few companies tat puts the rpo codes in the truck for anyone to decode. There should be a sticker on the inside of your glove box with all of the option codes for your ride. Scan through the codes until you come across one of the ones above, and there you go! I'm not afraid of math, but I'll only use it as a last resort!
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