Anybody know how to service one?

Depends on the age of the hub and what has happened to it. This is the manual for 2009 hubs, you can get others by searching the net.

http://www.tech-mavic.com/tech-mavic/technical_manual/data/docs/themes/2_13.pdf

Getting the freehub off is quite easy with the right allen keys, and you can get replacement pawls etc. from your local Mavic dealer, or from Hubdoctor in the US.

The plastic bush can wear and the pawls do break, but the freehub bodies themselves are pretty much indestructible as they are steel

Aah excellent, it’s the 2012 crossmax st I’ll have another search online

Login for their manuals:
http://singletrackworld.com/forum/topic/mavic-tech-page-login

I had this problem on my 2009 Crossmax ST for a long time until I recently discovered Mavic recommend the seal interface with the freehub should be lubed with Mineral oil not grease.
Used some brake fluid mineral oil & now its very slick!!

A 2009 ST and a 2012 ST are totally different hub wise. Have you checked the preload of the bearings? Maybe too tight?