There were three Sterling axles installed in Ford trucks from the IDI era to match the three different truck configurations.

1. Single rear wheel truck that left the factory with a bed installed, frame humps over the rear axle, 37.5" frame.
2.Dual rear wheel cab and chassis F350, 34" frame rails, straight frame over the rear axle. Same axle as the SRW truck, spring perches are 4" closer together thant they are on the SRW axle.
3. Dually F350, 37.5" frame rails, frame hump over rear axle, axle housing 4" longer back plate to back plate than a SRW axle.

If the truck left the factory with a bed installed it has a 37.5" frame, you can only run duals on the dually axle housing under that truck.

The SRW truck and the cab and chassis truck use the same length axle. The cab and chassis can run duals on the axle because the frame is 3.5" narrower. You can't use a cab and chassis axle under a factory installed bed truck unless you move the spring perches. Then it would be exactly like the axle you have under the SRW truck.

Hope this all makes sense, I had a long day today and it sounded good to me, I think.

we got one and we ran spacers to get awy from frame rail

With the weight I haul, spacers are not an option.
Wheel bearing failure would be the next thing on my list.