This will show you the different options for 6wd.
http://www.rctruckandconstruction.co...read.php?t=128
The truck tech forum is full of info for newbies to the hobby.
http://www.rctruckandconstruction.co...er=desc&page=3
It's probably shown in a picture somewhere in the truck tech forum as well of how to lock a diff, but the simple explanation is the 3 small gears within the differential itself, take one gear of the cross and fit it between the other two. It's easily removeable and change back to an open diff.
I use Shoo Goo in some of my diffs, works great for a limited slip diff. I usually lock the rearmost diff and add Shoo Goo to the other one (or two if it's 6wd). Too much bind in the drivetrain to lock them all, driveshafts and pinions won't last long and perhaps strip gears in the trans too.