I keep an old piece of plywood for this job. I lay the plywood in the driveway, put one of the cooking racks on it, and pressure wash it. In about 30 seconds it knocks off all the crusty stuff and leaves you with racks which still have enough oily residue to not rust, but not enough so it will go rancid and stink.
I use a very very cheap electric pressure washer. In fact, I paid $20 for it (used) on CL. I only use it for this task. Really, it works great.
Before I got this, I cut the bottom 5" off a 55 gallon drum to make a round pan. I used to put that pan on my turkey fryer burner full of water, with a few tablespoons of dishwashing detergent, and put in all my grills at once and bring the liquid to a boil. Then I'd rinse the
grates with a stiff spray from a hose and stand them up in the sunshine to dry. That
effectively degreases them and sanitizes them, but it leaves you with a godawful nasty
soup, plus you have to reseason afterwards. Worked OK, though.
seattlepitboss