It's an easy analogy to help understand what happens when engine braking. If you start to consider everything moving it makes everything difficult to understand. If you picture the engine braking enough to stop the Jeep entirely it's easy to see when the rear diff unlocks the wheel with traction rolls down the hill as the opposite wheel without traction spins backwards. When everything is moving down the hill with the driveshaft spinning that really simple concept becomes difficult to see.
OK, so the driveshaft is locked like the tranny is in park or gear with the engine off. There isn't enough traction so the jeep is sliding down the hill.
Say 1 tire is on ice and the other on pavement. The tire on pavement rolls down the hill while the tire on ice spins backwards. There is little resistance to going down the hill. The spinning tire on ice provides very little lateral resistance. The rolling, non-spinning tire on pavement has great traction and resistance to sliding sideways.
If the axle was locked both tires would be sliding. The tire on pavement would resist rolling down the hill (it's locked and not rotating) so there'd be less tendency to slide in the 1st place, but once it is slipping it would have less resistance to sliding sideways than with the unlock diff.
I'd have to go with unlocked in this scenario.