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1994 gmc 1500 4x4 pickup

My truck speedometer is acting strangely. The inside meter indicates 20 to 60 mph when i first start engine and am not movine. after I begin driving in the beginning it will indicate 10 or 10+ excess mph and some times it will indicate close to the correct speed. (I use a gps to check) If I put the cruise condtrol on then the truck will control at the correct speed but the indicator seems to respond to the action of speed control module indicating all over the place. I have replaced the speed sensor on transfer case to no avail. The odemeter seems to indicate close to accurate. What is the problem? Is it electrical, like a ground or broken conductor?

help please.