How have you stripped the ball diff? Is it just plain wear on the balls and plates or is something else going on. How tight do you crank it down? The Manta Ray diff should be at least as tough as the TA03. They're the same diff, but the Manta Ray is just bulkier.
If you're not running TCS rules, then you could try the 3Racing oil filled gear diff with some 300,000+ wt silicone in it. You could run a locked diff too, if your club allows.