The pinion seal is frequently replaced (mine went in my 90 745 at 55k), and the fact that the oil is down an inch is a good indicator. If you have actually fried your wheel, pinion or diff bearings, then you have a much bigger problem than just the seal. From past experience with rear-wheel drive cars, I have found that rigorous attention to driveway fluid analysis will almost always catch these problems before they become catastrophic. I take a hard look at the driveway or parking lot every time I back out, and any fluids that weren't there when the car was parked are examined closely. I have caught all of the leaks in my pinion seal, trans lines or pan gasket, radiator, p/s rack, and engine front seals this way. My wife also now does this, after she noticed "lots of green stuff" under the car just before driving away. Another good idea is to keep the engine compartment, transmission housing (especially the rear) and differential reasonably clean so leaks will show up more readily. Use a self-service car wash with the "tire cleaner/degreaser" to clean them twice a year.