1991 GMC K2500 Without HD Auxiliary Cooler; With Axle Ratio 4.10 (RPO code GT5) / 4.56 (RPO code HC4); Position: Inlet (Upper);. 1993 GMC K2500 Without HD Auxiliary Cooler; With Axle Ratio 4.10 (RPO code GT5) / 4.56 (RPO code HC4); Position: Inlet (Upper);.