I'm at 50K and the jam nut on the output of the Atlas was leaking a bit because it wasn't torqued to the proper specs, but otherwise everything seems fine.
I have been told that factory installed 4x4 on, say, trucks, is among the last things to fail.
That is a pretty dubious statment. For example I can wreck the 4x on any truck or Jeep in 6 months by doing one simple thing: snow plowing. Also Ford Superduty axels have a lubrication problem that causes them to fail because most people don't lock their hubs ever. I drive my F250 with hubs locked in 2wd at least once every two months to lubricate everything.
The SMB 4x uses the most bomb-proof parts available in my opinion (ok well the hubs are the exception). The Atlas II's only weakness in my opinion is that it's not guarenteed to flat tow over 300 miles without starting the engine.
I went out of my way to get the SMB 4x components because I believe they are over engineered for the van. The turn radius and 2wd low range have made it worth it already.