Ok Dave,
A fiber glass shower stall knocks a tile grout leak right outta da ball park.
To remove the old drain first check for a "O" ring if so follow the directions on this link,
http://www.plumbrite.com/?page=insta...765d5078b6f96d
if you can't find a "O" ring then your drain is cemented to the PVC stubup.
This will be a hassle to remove. If you don't wish to disturb the ceiling you must rent a set of inside PVC cutters, Purchase a small can of PVC primer and a small can of PVC cement. You will also need a 2" PVC DWV coupling and enough 2" PVC pipe to replace the pipe you're cutting out. Make the cut the same length as the one you cut out with the drain. Now follow directions to cut out the drain that WingTite has on the right side of the page. once the old drain is out, prime the coupling and the new stubup and glue them in. You are now ready to install the WingTite drain.
However, there is a method that just may save you all the hassle of then above.
MAKE VERY SURE THAT ALL SURFACES ARE DRY!! And then take and caulk the lip of the drain with silicon jel. It doesn't sound like the leak is all that serious so it might save you a load of work. good luck, Tom