You can do it with the trans in the car, despite other people's claims. I've actually done it so I know it's possible.
You'll need to drop (but not remove) the cat-back, and remove the h-pipe. Remove the O2 sensors first.
Drain the trans and remove the driveshaft.
Put a jack under the trans and remove the trans crossmember.
Unbolt and remove the speedo cable housing and unplug the other electical connectors
Remove the shifter and drive the 3/16 roll pin out of the shift ofsset block.
Remove the bolts holding on the extension housing.
Wiggle the extension housing off the trans. It's a good idea to reach up and hold the shifter offset block from sproinging up when it comes off the shift rail.
To reinstall the housing, put it up loosely in place, then put the shift block spring and check ball in place on the extension housing. A little vaseline or light grease will hold it in place.
Now either with a helper or by reaching up and around from underneath, hold the shift block down so its hole aligns with the shift rail and seat the extension housing.
Snug it down temporarily and make sure the shift block is in place properly. If so, drive the roll pin back in, go down and finish tightening up the housing.
Leave the shifter off until after you install the driveshaft, you can easily fill the trans through the top when the shifter isn't in the way.
The hard part is over, now put it all back together. Use lots of antiseize on those exhaust bolts and don't be afraid to use new hardware, you'll kiss yourself the next time it all needs to come out.
Use loctite on the driveshaft bolts and reef them down tight.