I used to watch a YouTube channel where the guys would build custom off road rock crawlers where they were constantly swapping frames and axles between Ford and Chevy's and all sorts of other things. One of the guys made a comment where they said that the frames for dually's are narrower than single rear wheel frames and that the spring mount position on the axle is different between them. I do not know if the frames are different or not and can't confirm any of this data, but a couple items to consider before purchasing any axle:
- Width of the springs used compared to what's on your van, and also the width of the spring mounts to accept the dually springs on your frame (if changing springs).
- Distance between spring mounts on the frame (if changing springs).
- Location of the shock mounts on both axles, and also the spring mount location on the axle.
- Length of the driveshaft and U-Joint mounting/configuration.
There's probably another item or two, but all I'm saying is that this may or may not be a simple bolt in installation and depending on which chassis the donor axle comes from, there may be more work than if it comes from an Express chassis.
When a truck is constructed as a cab and chassis it's not uncommon to design to some kind of gentlemen's agreement for a frame width that most of the manufacturers and upfitters use, a frame that is actually narrower than what a manufacturer chooses for their bed-equipped pickup trucks. Due to that narrower frame, a narrower dually axle can be used. Since pickup trucks usually try to lower the bed floor as much as possible leaving wheel wells that protrude into the bed, they want the frame to be as wide as they can achieve at the rear axle to allow for the widest possible load between the wheel wells.
I have not personally witnessed a truck manufacturer go with a different frame width for a given weight rating (ie '150/1500', '250/2500', or '350/3500') for their factory bed equipped trucks when SRW or DRW, but admittedly I haven't seen all of the brands from all of the manufacturers either. But it would follow that if the SRW and DRW beds have the same inside bed sheet metal, then a cab and chassis narrow dually axle wouldn't be wide enough for either the frame or the wheel wells into the inside of the bed.
I've found some supposed claims of "Bible of fourteen-bolt rear ends!" sort of sites but they still don't seem to be truly comprehensive.
From the information I've found, supposedly a GM truck platform dually has a rear axle that's 72" wide, while the axle in the Express (not necessarily the cab and chassis!) is 70" wide. That difference is minor enough that if a very small spacer is all that it takes to clear the frame and fiberglass RV wheel wells towards the inside, that I would be willing to live with. That wouldn't be putting so much strain on those wheel bearings where the duty cycle of an RV would roast them prematurely. But if it takes a much thicker spacer, that would be a problem.
The guy that did the one here used and Amecrican Axle full floating rear from a later year Express 3500 single wheel per him selling another of the same axle he had. It was a single wheel axle so he must have used spacers in the rear.
I'll need to dig in to the model year differences of the production run. His was something like a 2012, mine's a 2006, there could be important differences in how GM built the base vehicle. Or his research might have not gone back as far as my van either, and there could be no differences of distinction. The latter wouldn't surprise me at all if GM expected to sell these to upfitters generally, anything that they could leave the same would be better for their customers unless there was something wrong with what they had.
