It might not be environmentally friendly, but this one works wonders.

Take some power washer soap, and mix it with about a half gallon of diesel fuel. Get an air gun with a siphon hose for your air compressor, and use it to spray down the tractor. A light coating will only take about quart of the mixture, so it's not like your using a lot of fuel. Besides, you're going to be washing off all sorts of grease and oil anyways, so a little fuel isn't going to change things much.

Let the stuff soak for about 15 minutes and then go at it with the power washer. You'll be amazed at the way it comes off. To prove it, try power washing an area without the mixture on it and one with. You may have to repeat a couple of times if the grease is really thick. It will save a lot of scraping later on.