First, you need to make sure you're not leaking from the speedo cable/speed sensor area. This is a pretty common place to leak on these cars.
If that is OK, and you're still leaking...
The slip yoke part of the driveshaft slides into a cylindrical/collar style bushing that is pressed into the tail housing of the transmission. This should be a very tight fit and restricts a lot of non-axial movement between the driveshaft and transmission. if you have an imbalance in the driveshaft or a really loose trans mount (that causes extreme angles between the driveshaft and trans), it is possible to strain this area and cause damage to either the bushing or the slip yoke surface, or both. This, in turn, allows more movement that can cause damage to the tail housing seal and let fluid leak out.
To check if you have any damage to the slip yoke, you can pull the driveshaft and run your finger long-ways across the slip yoke surface. If you feel any waves or grooves, even if you can't see them, you may have damage that is causing a leak.
To fix this, you need to pull out the old bushing and press in a new one, and you have to replace the slip yoke (or the entire driveshaft) at the same time. Replacing just one or the other won't work, you'll just end up damaging the new part. Replacing the trans mount at this point is a good idea, too.
I know this because I've been through the same thing myself. I used it as an excuse to buy an aluminum driveshaft for the car.
