leaking from the shaft drive breather boot on Yamaha motorcycle?

Usually this occurs (and it's quite common) when the final drive (differential) is too full. If you have had the bike serviced latley, i would suspect this. Barring that, the breather may be slightly plugged up, not allowing it to breath properly. Too much pressure build up through too little of a hole would do this as well. I would drain the diff and start over again, putting in the proper quantity and see if that solves the issue. Should be changed every 2 years or 15,000 kms anyways.

From what i remember, the breather units are press fit into the aluminum body. Maybe it is loose, but i doubt it. Give it a wiggle and see. New units are around $10.

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If There Is Oil Coming From A Breather Plug About The Only Way For That To Happen Is From Over Filling.

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You might try the recommended 80 weight gear oil. When the oil is cold and stiff it can be forced out of the breather because it doesn't "flow" well. you can use 75W90 Mobil1.

Also, be sure to fill it with the bike on the centerstand. Otherwise you CAN overfill it slightly. A combination of cold temps, slight overfilling and too heavy oil could create your problem.

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Is there any damage to the breather cap? Is it properly vented? A clogged or partially clogged vent can allow a lot of pressure to build up in there. A damaged one could allow air passing over the vent to create a vacuum and draw fluid out.

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Check final dive seal, they get worn out and cause this problems.