Verschuren,
I'm assuming yours is a 450 series GMC. There were a couple different rears available. Single speed, two speed in a couple different weigh ratings. Are you looking just for ratios or parts numbers and swap info?
If it is a 450 there should be a data plate inside the glove box door that tells what equipment the truck was born with. If you post what it says we should be able to dig into this a little deeper.
Scott
