I've had luck cleaning heavily greased metal with an oxygen acetylene cutting bar. I know your first reaction is to say it won't work and I'm off my rocker, but this is a careful process, not a melt down. The idea is to burn off the grease with the oxygen rich flame you get when you push down the cutting lever, and keep the flame moving. The grease will pop off the metal, trailing flame and smoke, so wear eye protection and heavy clothing. Do it in a place where you won't start a fire, and have good ventilation. You don't have to 'preheat' the metal like when you are trying to cut through it, just put the flame on the grease and pull the oxygen on....poof, it's off the metal. It will even work on old paint and years old automotive grease or oil. Be aware that old paint and grease fumes can be bad for your health. I've used it on aluminum barbecues and on cast iron grills. The metal doesn't even really get hot, as you keep the flame moving. The ash and chunks burned off will make a mess, so don't do it where the stains will matter.
It may not be the best way, but it worked for me. You might try a little scraping or pressure washing first, just to get off the loose stuff.