98 Astro Pinion Seal Replacement

My 111,000 mile 98 Astro appears to be slinging oil from the pinion seal on the front of the differetial (there is oil on the fuel tank in that area), it is not dripping or making a mess of my garage and I replaced the oil in it recently. My Haynes manual does not have a procedure for the seal change. Should I attempt it and if so what is the procedure? (I am a former military and aircraft mechanic)