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Mercedes Benz Chassis grease. Sticky stuff, only available through Mercedes dealerships (go along and give some money for the coffee to the mech´s....:D and take a small can with you......)
A good lithium based ball joint grease is recommended. GM dealers can obtain Chassis Lube #12377985. Just be careful, the part number is for a case of 10 tubes. You only need one tube for the gear.
As mentioned previously, don't overfill the gear. You need some air pockets inside to allow for expansion of the grease because of normal underhood temperatures.
Jim is right on. I over-filled mine last summer with chassis grease and it leaked a bit. I took some out and problem gone. What prompted this was a post to just test the level. Well, there was no level. The box was dry or congealed. Check it for safety's sake.