[rwgregory]

Sounds to me like you are looking at a Dana 44 rear axle with a 46 tooth ring gear and a 13 tooth pinion that would calc. out to a 3:54 rear end ratio. What it is doing under your truck is probably pushing it down the road with power from the engine and trans.
All kidding aside, GMC's came stock from the factory with Dana rear axles. 1/2 tons had Dana 44's and 3/4's came with Dana 60's.