I am working on cleaning up my diff while its out of the car so I can paint it and was wondering what everyone liked to use as a degreaser.

Matt,

I've had good luck with the following:

Castrol Super Clean, Brake Kleen, and Gunk Engine Brite which comes in a can (foam or gel form) & some warm water & a soft brush.

If the surface still feels slick wipe it down with a pre-paint prep to remove any residue.  Give it a scuff with a scotch-brite pad or fine sandpaper before priming and the paint will stick nicely.

Anthony

Brakleen gets my vote...there are a number store brands that work about the same.

JR

Easy Off oven cleaner will take off everything, including any paint that's still left.

Any of the above products will work fine. All of them have some conditions, tho. The GUNK is kinda greasy, like diesel, but it cleans well. The BrakeKleen works well, also, but may not get all of the thick crud off. Super Clean, the same. Easy Off is the best at getting everything off, but it will damage any seals that are still on the diff.
I would go with the Gunk Engine Brite, and a good stiff brush, then take it to a car wash, and blast the residue off.         my 2c

I use K1 Kerosine for internal items (it's pretty mild) and Easy Off (it's more wild)for external.

I like to use brake cleaner to clean up greasy stuff. I used it on my door lock mechanisms and it cleaned them like new. You will still have to help the product out by scrubbbing in some places. Most of the grease will be gone though.