Is the spur gear turning when you rev the engine?

If it is, look at the drive shafts going to each rear wheel. Are both of them turning?
-If one shaft is turning but the other one isn't then you probably have a broken roll pin in that axle (the diff is diverting all the power to that wheel since it's the path of least resistance).
-If neither shaft is turning then the problem has to be inside the transmission, most likely a stripped gear.