Sounds like you may be on the right track, see if your son can look underneath there and tell you if one of the axles looks wet or moist as if one of the seals has been compromised.
I had an axle go bad once in a Mercedes and it only manifested itself at high speed when the driveline got hot. Then it was a hard rythmic banging noise. I could stop and cool the car down and the noise would go away until the driveline got hot again. Axles can fail differently though. I would personally recommend new Mercedes axles first. Second best would be CVJ rebuilt axles, which is what I used because new Mercedes axles are really expensive. So far they have been great.
Another thing you could look at are rear sway bar links? Those are cheap. Easy to see if they are shot. Those cars have a multilink rear suspension so look at all those links and determine if there is something amiss. Rear wheel bearings? Last thing would be differential but not likely.

__________________
'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)