I assume you are talking about a K. I need details of the truck: year, model, transfer case, etc.
Give me an exact number for how slow, and I will look up the ACDelco ratio adapter part number. Go out on the interstate. Note your odometer reading (including tenths) as you pass a mile marker. Drive ten miles. Note your odometer reading (including tenths) as you pass that mile marker. Subtract the two. You should have something like 7.1 miles indicated. That let's me get the right adapter number. Then you just look up the GM part number on the net and buy it.
The keys are here. You need the HR124 and the HM146. No internet ordering, you call them on the phone and they ship them out same day or next day depending on whether the UPS truck has been and gone already today. They are about $10 total.
http://www.texasindustrialelectric.c...on_Adapter.asp