Mavic Freehubs contain a plastic bush (toroid) between the nose of the rear axle and the freehub that gets word down causing play in the cassette. I was told to tighten my cassette, or add a spacer. I ended up cracking the rear freehub due to tightening it too much. I don't think a spacer is necessary except in the case of 10-speed Campy cassettes.
Freehubs, and replacement bushes, can be replaced/fitted to Aksium wheels using a 5mm hexagonal wrench (allen key?) on the drive side and 17mm cone spanner on the non-drive side (instead of a 10mm hex spanner on more expensive versions) 
Be careful not to loose the springs and "pawls" when taking the freehub off.
The Shimano fitting freehub on pre-2013 models is the M10 M40592 which weighs 90gms (Campy ED11 M40591) From 2013 they are Shimano HG11 308 711 01 and campy ED11 308 713 01 respectively. The new ones will work on old wheels using the "shouldered washer" (instead of the flat spacer), included.
Information from Mav on the procedure hubassembly difference between 2013 and previous hubs, [http://www.mavic.com/sites/default/files/download/FTSL_0.pdf overal procedur