Check the tailshaft bushing and seal for wear. The rear seal and bushing of the T5 transmission contact the slip yoke to prevent the tailhousing from leaking fluid during operation. If the tailshaft bushing has any pitting or wear marks, it can create a gap in between the slip yoke on the driveshaft and the tailshaft bushing/seal, which may allow transmission fluid to escape when the car is moving. It's also a good idea to check the surface of the driveshaft slip yoke. The outer tube of the yoke should be smooth from one end to the other. If there is a groove cut into the yoke the same size as the length of the tailshaft bushing, change the yoke and tailshaft bushing to ensure there are no leaks.