I've got a Deore XT rear hub (FH-M756-A) on my tourer, and twice the freehub has let me down. The first time it was full of mud mixed with grease but I managed to get it working just enough to ride home. I replaced the whole freehub body - that was a year ago.
I recently serviced the new one. It was pretty muddy inside, so I cleaned and regreased it. That failed on me yesterday, luckily near a station so I only ended up with 11km to walk/roll downhill.
Clearly the seals aren't up to my (mainly road) riding despite being MTB parts. Rather than rebuild the wheel on a new hub, I'd like to upgrade the freehub body, but is that possible? I can't find a compatibility matrix, and Sheldon says "Most Shimano Freehub bodies are interchangeable" before listing a lot of exceptions. In my particular case it's QR versions, of which there are many - so is it possible to look at the specs and work out whether a given body will fit? Even better, is it possible to work out which would have a good chance of keeping the mud out?