News coming out of Overland Expo East this week (Nov 2018) suggests that Iglhaut, one of the big German 4×4 Sprinter conversion companies, will be accepting orders in the USA soon. That means true 50/50 split full time 4×4 with locking differentials. But it isn’t cheap.
It works like this: You buy a US-spec 2WD van and it gets sent to Germany. After around six months you get back the same van with the conversion, 37″ wheels, skid plates and a couple of other off road features. The cost, including your purchase of the van, is around $130k.
Iglhaut retrofits a drivetrain based on the Mercedes G-Wagen to their European conversions. No reason to think their USA offering would be any different.
Their site is a placeholder still, but it looks like they’ll start taking orders soon.