a differential is a mechanical device used to transmit engine rotation , having passed through a transmission ( various gearing to multiply engine torque) via a drive shaft
to the drive wheel of a vehicle. Open Diffs are a series of gears that permit the right rear wheel to drive the car and the left read wheel to rotate in the opposite direction. This is why only one tire leaves a ' patch of rubber' when yo udo a burn out.
if yo uever yjacked up the rear end of one of these cars and turned one rear wheel, the other wheel will rotate in the opposite direction...
A limited differental has clutches tha tlock up both wheels when going in a staight line..GM called it Positraction, Chrysler called it Sure grip..etc..the clutches are required so they can unlock the inside wheel when making a turn permitting the outside wheel to drive aroubnd the corner. If you ran a spool ( no diff gears , both axels locked in drive mode) you would spin the car out going around the turn. A 4 x 4 is simply two differentials..one in front and one in the back,,both driven by drive shaft from a transmission via transfer case.. that when actuated will provide drive to all 4 wheels..in my opinion, doubles the weight , parasitic drag and kills MPG, a ral bear to work on and ...a must for soccer moms afraid of that really bad snow storm that happens once every 10 years...oooooo!
i did not see the movie so can not comment on question.