http://www.christini.com/
I would really like to take one on the dirt, to feel the powertrain pulling with both wheels, and it disengages on the fly.. Check out the AWD technology tab to see how this works.
I remember a 2x2 trail bike with fat balloon tires
from the seventies, but I don't remember the manuf. name.

In no competition is the Rokon's, but a true trail machine.
http://www.rokon.com/index.php?p=1_4_Trail-Breaker