Could be a problem with the Transfer case control module, (TCCM) or the encoder motor on the transfer case, or a corroded connection somewhere in the circuits. Two common problems are corrosion at the TCCM connector, or a faulty encoder motor. Best to have it checked professionally. 4WD on these vehicles should not be used on dry roads or damage will ocurr. If the transfer case is in 4WD low range, do not exceed 30mph or engine RPM will be excessive and can damage the engine.