Being the unfortunate owner of an sl-600, and previous owner of 380 se(AMG engine) 500 sel and 560 sel (126's), I can assure you repair costs for the MB versions later than the 560sl are an order of magnitude higher.

It's really unusual for the axle shafts to fail in a way that causes the noise you describe. Very unusual for a driveshaft to fail in a manner that makes noise only at a specific speed.

Please note--both the driveshaft and rear axle shafts rotate at a speed directly related to road speed, not engine speed--they do not 'see' the gear at all. If you have a noise that is specific to a GEAR and engine speed--it is not the driveshaft or rear axle.

There HAVE been some instances of reverse clutch disks in the automatic transmission rattling at 1500RPM. There is a TSB on this. It will do this in all shift position except 'R'

Fix is renewing the inner plates of reverse clutch B3 and setting release clearance to minimum value.

I HAVE seen a loose hub attachment to the axle shaft though--and it will cause weird noises.