The 2-speed units are just overgrown versions of the pickup rear ends, when it comes to the pinion flange and seal. If it's got any front-to-rear slack in the flange, it's probably got a loose/worn pinion bearing, and replacing the seal isn't likely to help for long. The first thing to do is disconnect the driveshaft, and check for fore-and-aft play in the flange. If it's snug, but leaking, go ahead with the seal change.
You'll need to jack up at least one wheel, or pull the axle shafts out of the hubs (preferred method), so you can do a "turning torque" check before disassembling the yoke. This is important so you can get the correct bearing preload when you reinstall the yoke after changing the seal. With a big socket adapted to an inch-pound torque wrench, check the torque necessary to turn the pinion flange. It should be somewhwre in the 20 to 50 inch-pound range on the big rear axle. (Just guessing here, but they're stiffer than the pickup third members.)
Remove the pinion nut, (Probably VERY tight) and slide the yoke off the input shaft. If it's tight on the spline, it might take a little BFH work. Pry out the old seal, and carefully install the new one, with the sealing lip facing inward toward the gear oil it's trying to seal. Lube the lip seal with a little gear oil, never install one dry! Reinstall the yoke, after inspecting the seal surface for burrs or wear. Sometimes there will be a ring worn in the seal area where tht old seal was running. If so either replace the yoke, or install a "speedy sleeve", a thin, flat ring that goes over the seal area to smooth it out. Most bearing-supply places will have them, or be able to order one. Retighten the pinion nut until the turning torque is the same as before you removed the flange. This might take a LONG wrench and a lot of effort! Reassemble everything, check the lube level, and drive on!
Jerry
