How do I clean the inside of a freehub?
That particular Shimano freehub can be disassembled, but it is quite a job to get it back together afterward. There are around 80 2mm bearings in two different locations in the freehub, and a skilled and practiced mechanic has roughly a 60% chance of opening without losing parts, and successfully getting it back together. 

The good news is, there is a tool to lubricate the inside of a freehub, without disassembling it. It's called a Freehub Buddy.



The photo shows the Freehub buddy, next to a freehub with a cutaway so you can see the rows of bearings I mentioned earlier, a tube of soup lube, and 2 new reusable seals.

It's made by Morningstar tools, and works very well. You will also need the proper lubricant, and new exterior seals for the freehub. Both can be purchased from Morningstar tools as well. 

The other option is to simply replace the freehub, which should be readily available at your LBS, for about $20. 

I hope that helps.