1988 GMC K3500 Without HD Auxiliary Cooler; With Axle Ratio 3.42 (RPO code GU6) / 3.73 (RPO code GT4); Position: Outlet (Lower);. 1989 GMC K3500 Without HD Auxiliary Cooler; With Axle Ratio 3.42 (RPO code GU6) / 3.73 (RPO code GT4); Position: Outlet (Lower);.