I have the front belt on my 05 set quite loose, its probably got that way from the spool up front, the rear belt is a bit tighter. Providing the belts dont skip their teeth, the looser the better usually, it's a bit like the spur and pinion gear mesh, as it never pays to have them set overly tight.
Diff's, well as long as they dont slip you're pretty much good to go. hold the diff and one wheel whilst trying to turn the other wheel on the same end, if it turns with little resistance, its too loose. It should take a "reasonable" amount force to turn the other wheel, I can't really describe it over a forum.