When I first started the direct to diff stuff I was going to use some sort of viscous coupling between the outrunner and the driveshaft. Similar to what you are doing now, I wias going to drive into one outdrive of the diff, and output to the rear diff from the other outdrive. If the fluid was thick enough the entire diff would turn, but if sufficient load was placed on the wheels just the input and the case would rotate. I eventually scrapped the idea due to weight concerns, as I would have to run 2 setups for 4wd.