With a manual, just put it in neutral and start yer towing.

With an auto, need to disconnect the drive shaft as mentioned.
Why do you need to pull it out of the trans though? Just disconnect the
rear u-joint and tie the shaft up out of the way with bailing wire. Make sure it can't slip out of the trans though.

 

you certainly could do that, but if your towing it for a long distance, i would not want it hanging where it may fall. and you "could" just put it in neutral, but i would NOT suggest it, incase it were to slip into gear when you were driving, not a good day. i would rather be safe than sorry