For the 1973-1991 model years, GM primarily used Dana 70 series rear axles in 1 ton, "square body" trucks with dual rear wheels and 1 ton vans with either single or dual rear wheels. For the trucks, only Dana 70HD (heavy-Duty) axles were used. Vans could have Dana 70B, 70U, or 70HD axles The axle Bill of material number which is either stamped on the back of the right axle tube or on a metal tag retained by the diff cover bolts can be used to identify the axle version you have. If the numbers are missing or illegible, you can pinpoint the axle model by pinion and differential bearing cup size:
Dana 70B:
Inner Pinion Bearing Cup OD is 4-1/8"
Differential Bearing Cup OD is 3-13/16"
Dana 70HD
Inner Pinion Bearing Cup OD is 4-1/8"
Differential Bearing Cup OD is 4-1/8"
Dana 70U:
Inner Pinion Bearing Cup OD is 3-1/2"
Differential Bearing Cup OD is 3-13/16"
