Well, as far as your diff is concerned, it sounds like you've got it way too tight. It should be tight enough that it doesn't slip but still allows your tires to turn independently in the corners. With as tight as your diff is, you're probably having issues in the corners with the rear end wanting to come around on you all the time. When it's too tight, it acts like a spool and the inside tire breaks traction giving you only 50% of the grip you would have normally then causing the outside tire to break loose as well under power. To check the tightness of your diff, take it out of the tranny... Lock the outdrives with 2 hex drivers or screwdrivers of allen wrenches or whatever. Then try to turn the diff gear with your fingers. You should only tighten to the point that you can't turn the gear anymore. Any tighter and you start to lose diff action and you get to the point of a spool.
On the slipper.. The sound you need to listen for is a very high pitched squealing sound. I set mine so I have full power on take off, but it will still slip on landings off of jumps and bumps to protect the drivetrain. If you nail the throttle from a dead start, it should slip at the most for the first foot of the launch. Anymore than that and you will lose your punch out of corners and it'll be hard to time the jumps properly without the power to the ground to get what air you need. I run a 12x2 in my T4 and it doesn't slip at all on take off but it will slip a little when I land from a jump with the throttle on. A lot of people make the mistake of using the slipper as a bit of a traction control on take off. The intention of the slipper clutch is just to protect the drivetrain. The sudden shock created to the universals and outdrives and such when you land from jumps can cause them to fail. The slipper is there to absorb that. If you're running at a local track, your best bet is to ask one of the experienced guys to give you a hand with it. If you're just bashing in parking lots and such, take it to the LHS and see if one of the guys there can give you a hand.