You won't find the information on the truck it self. Most GM axles have a two letter stamp in the axle tube out by the brake backing plate. Most are on the passenger side of the axle. If you do fine the code, ( AX, AD, Etc...) you will need a shop manual for that year to tell what the code means. The best way to find out is to pull the cover and check the part numbers and date code. The ring gear should be marked with all the information, ratio, date, and number. If you can't turn the axles due to weight on truck, you may be stuck. You can't count the teeth if you can't turn it, and you read the numbers if they are on the back side.
Try taking a wire brush over the axle tubes and check for codes. At least get the engine and transmission type from the truck. If it wasn't special ordered, someone should be able to get close on the ratio.
As for fit, Posi-traction won't make any difference. It is a internal anti-spin carrier which the ring gear bolts to. Joe
