I'll be changing my driveshaft next week. When I swap it out, should I expect fluid to come out of the tailshaft of the tranny? If so, how much? Should I go and buy some fluid to add to the tranny? Thanks.

 

You may loose a little but not enough to make a difference. Lift the car at the rear to minimize this. Put a container under the trans. if you want to see how much comes out. Usually it looks like more when it's on the ground.