I seriously doubt there's any real problem with the diff, you'd know something was really up with the diff whenever you turn a corner, the tighter the turn the more noticeable it would be, and it would be VERY noticable too. That's really the most likely indication that there might be something wrong with the diff, turning corners! Doesn't hurt to have the lube level in the diff checked though, could probably have that done quick and relatively cheap at most any lube place, your problem may very well lie with a sticking brake caliper. That "run out" issue you described albeit very confusing sounding, on what sounds like the "can" on the axle halfshaft probably isn't a real issue either. Those are just "cans" with lots of clearance room and filled with grease. As long as your boots aren't torn (they aren't torn, are they?), your axles are mostly likely good to go too. Focus on the brake calipers and just plan on rebuilding them just as a part of regular preventive maintence anyway. Replace the pads when your done AND the rotors, Mercedes rotors don't allow for a whole lot of extra material in the rotors for machining, they are considered a replacment item when doing a brake job, that will eliminate run out in the rotors and another potential for effecting the way the brakes release. Also just as importantly are the brake hoses, if they are the original ones, the rubber inside can be deterioating and can restrict the proper flow of the brake fluid and could impede the pressure relief when you lift your foot of the brake pedal, again replacing the hoses is cheap, easy and part of regular preventive mantience. See if that doesn't help before going off the deep end into axle replacement or opeing up the diff. (That is, unless your axle boots are torn, in which case, you know what's next!)