I didn't bother reading all of the posts, just skimmed, but the general gist is right.
WD40 is BAD. That is why it sounds gritty. WD40 disperses and breaks down all lubricating substance, yet still retains the grit. So it is bone dry and gritty in there. You now need to pull it all apart, give it a thorough rub with a cloth, getting as much WD40 off as you can, then use a lubricant, degreaser, lubricant as the first application of lubricant will remove some of the WD40.
Now, to service your freehub, GET THE TOOLS. That is number 1 rule in maintaining a bike. Get the tools to pull the parts you need to maintain apart, you will use them again. Use a book like Zinn and the art of Mountain Bike Maintenance (or road, they are great). If you can't get the tools, go to a bike shop and pay them to do it.
OK, depending on what freehub you have, you should be able to pull it off with a combination of allen keys and cone wrenches. If you have a full set of allen keys (1-6,8,10,12) your life will be easier. Most rear hubs use 15 & 17mm cone nuts. So, lay out a clean cloth, and take it apart over that, to catch anything (helps if its white, though it won't stay white). If its a sealed unit (sealed bearing hubs) these are quite self explanatory. An allen key in one end of the axle, a cone rench at the freehub side, normally. DT us a press on O ring end cap, which is easier. After you have the freehub side endcap off, you can remove the freehub (it'll slide off). Clean the drive ring (serrated ring inside the hub) with a cloth, and the freehub pawls (tooth things that spring back when you press them in). then use some lube like Finish Line cross country (grease can stick the pawls down) or go lighter oil dpepnding on how uch you want to maintain it and how loud you want it to sound. re assemble in reverse to dissassembly.
Now, Shimano use a 12mm hex key to remove their freehub. I have more limited experience here, so I won't try and tell you what to do.
And if its a loose ball hub, when you put them in, give them a good coating of grease, they'll like you more for it. read up on how to adjust cones.