Conundrum - either I'm missing part of your question, or you are asking what happens to a front tire when the rear tires turn?

All the rear locker is doing is making the rear tires turn in unison, rather than just the fastest turning tire getting the torque...

So, the front tires just turn like they always would....and if you apply the skinny pedal, they turn faster...the open diff fronts act like open diff fronts...and the faster tire gets the torque, and the rears turn in unison...but faster.

If any thing, if the front tires are being pushed along by the locked rear, they have a better chance of turning at the same rate, as they are not required to supply as much traction, and so one of them is somewhat less likely to lose traction, spin, and steal all of the torque.

So, if front tire 4 is in the air, and the other three are on terra firma (or even on Xterra Firma...)...the rear locked tires push the truck foward, the tire in the air gets the front torque, and spins, in the air, at the same speed as the rear tires are turning...the front tire on the ground rolls along for the ride.

Is that what you were asking?