Here is a basic diagnostic chart from GM pertaining to the speedo being inop. If the cruise control and transmission are working ok I would suspect a bad instrument cluster.
Did you perform the Instrument Cluster System Check?
Go to Step 2
Go to Instrument Cluster System Check
Does the speedometer read the specified value?
Go to Step 4
Go to Step 3
Inspect CKT 817 (DK GRN/WHT) for the following conditions:
Is CKT 817 OK?
Go to Step 6
Go to Step 5
Did you find and correct the condition?
Repair CKT 817 (DK GRN/WHT). Refer to Repairing Damaged Wire Insulation in Wiring Systems.
Did you complete the repair?