Brian
If you have a bad leak, you're better off just fixing it (or having it fixed). I just replaced the shifter shaft seal on my '81, and it wasn't too bad of a job, (but then, I do have a lift in my garage). The tranny shop wanted $125 to do it. The only other place it leaked a little is out of the inspection cover drain hole, which usually indicates that the front seal is leaking. I certainly didn't think it was worth pulling the tranny for that, so when I refilled the trans. with fluid, I used the Valvoline Max Life, which is suppose to help with the seals. It's too soon to tell how much of an impact that will have, though.
Dick