I could be wrong but I think viscous couplers are an AWD thing.
I looked up the NP271 and 273, which are popular 4x4 t-cases for the Econoline platform, and I didn't see any viscous couplers there. Nor on the NP205, so I infer that 4x4 is straight drive.
Which brings up the point that AWD and 4x4 aren't the same thing. I mean, you probably already know that, but I have to remind myself sometimes.
2003 Astro AWD
2005 Tacoma Access cab 4x4
1999 E350 RB 7.3 "Al B. Tross" aka "Exxon Valdez"