Wel with it being 4x4 wouldnt the speed sensor be in the transfer case? I know its a diff style truck but in my 86 k10 the speedo cable plugs into the transfer case. Ive HAD to drive it some since the car had a flat and you can hear everthing turning in the rear like ring to pinion gear and like its just spining the carrier so like you said maybe just maybe the unit is bad only but my luck is never that good!