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Either the sensor in your tailshaft of the transfercase has failed, or your DRAC (little clear or yellow box with a 4 number code printed on it) has failed
1990-91 trucks have a digital speedometer instead of the cable used on the older trucks.
There's a sender thing at the t-case, and then the magic box (which I stink is the DRAC) which IIRC lives on the back of the cluster or in the dash up there somewhere.