Start by pulling your speed sensor off to see if the gear is still attached to it. I know it don't take much for these to fall off because they just snap to the sensor itself. And yes the differential is inside the tranny. It's what your axles slide into.
ok. so it's just a matter of taking sensor out and inspecting it. Should it be dry or will there be any kind of gear oil that should be coating it [since it is a gear]?
So the sensor should be small enough to fit in your hand right? and on the top of the tranny [it's the only one i've seen on the transmission with wires going to it.]. Drivers side to be exact.