I use a regular degreasser first then pressure wash all the bulk of grease and oil off. I use Zep Orange but any degreasser will work like purple power. Then I use a caustic degreasser that basically etches the aluminum and gets it perfectly clean. The best stuff was some I got from a friend who worked for a hvac company. It cleaned awesome and left the aluminum shiny. Most leave the aluminum a dull grey.
Only pressure wash a transmission if you will be taking it apart immediately. No matter how well you try to seal it up there will be some water that makes its way in.