A couple years ago before i took my car off the frame i jacked up one side as far as i could, put i ton jack stands and power washed the crap out of it. Then took a bunch cans of degreaser and brake cleaner and got under there and got all the rest of crud off the crossmember and tranny and rearend, it wasnt spotless but it came out pretty well. If you have access to a hot water or steam cleaner probably do even better. Very messy and dirty job, dont forget the eye protection, shit goes everywhere.