I need to know how much grease to put in my differential ? Do you fill them completely, or part of the way.

What truck? If its 1/8 scale then use diff oil. If its 1/10 then fill diffs part way.

Yeah guess it would help to know what its for lol, it would be a Tsunami, I opened them up last night and there was little to no oil what so ever in them, but maybe on the walls of it but thats all, what I'm using to fill them would be Multi-Purpose Lithium-Flex Grease.

That would work. I put 50k diff oil in mine. filled them up all the way. no leaks and it has been over 3 months since the mod. but you need to clean out all of the grease before adding the diff oil.