[icingdeath88]

Did the part that popped out stick into the transmission or out of it? i.e., is the splined shaft, the male part, that was separated on the transmission side or the axle side?

If it's the transmission side, then you'll need a spring pin to hold the axle onto the splined shaft. Have a look at the passenger side, to see the spring pin I'm talking about. It goes through the axle, through the splined shaft that sticks out of the trans, and back through the axle on the other side. Hopefully you just need that pin and everything will be ok.

If it's on the axle side, then that part is not supposed to come apart normally, and I don't know what happened or what to do. The rear axles just "pop" in and out of the rear diff. There is a little spring clip on each side that holds it in, which works because the rear axles don't have to change in length much at all, because those wheels don't turn. Maybe that shaft sticks into the trans with a similar mechanism. Look at the end of the shaft to see if there's anywhere a spring clip could grab onto it.