Hi Andy, drain the oil in the rear axle (or reverse it up a steep ramp), undo the four bolts holding the seal carrier and guard in place, slip it off the shaft, pop the seal out, get a new 40x72x12mm lip type seal from your local bearing shop, push it in the housing with a smear of geasket maker on the outside of the seal to assist fitting and sealing, make sure the lip on the seal points toward the oil side! Refit the seal carrier with gasket maker after cleaning the old gasket off.
Good Luck!
Aaron