For Sprinter,
Sportsmobile might have something or be willing to build something. Their factory is in Fresno.
The Sportsmobile site offers a link to the
Sprinter Cargo Van site, which says models are available in 4x4 but says nothing about DRW.
Also, you said "My single son is on the hunt for a 4x4 Sprinter ( orDodge, or International, or Ford) that has dual rear wheels, 4x4, transfer case, diesel engine in an RV format." What about a Tiger? They are available on Dodge and Ford these days, in addition to their more usual Chevy. Some can be had with DRW. Regular cab, extended cab, crew cab. Gas or diesel.
http://www.tigervehicles.com/