First thing I'd check would be the axle breather to make sure it's not blocked (unscrew it and give it a shake, you should be able to hear the ball bearing, if not wash it in a solvent of your choice), if it is blocked then when the diff warms the increase in internal pressure can blow the oil past the seal. Mine weeps very very slowly, nowhere near as bad as yours, maybe a teaspoon of oil gets out every 4-6 months but the hub cap traps it. As a temporary fix you could try pulling off the hub cap (it may be filled with oil so be prepared to catch it!), cleaning it up and then run a bead of hylomar instant gasket stuff round the edge when you refit it. If you have blown a seal there I'd also check inside the brake drum just to make sure you're not leaking onto the brake shoes too.