mY 90 740T also leaked around 80K, 9 years old or so.

Relatively easy to do yourself....just make sure the driveshaft flange goes onto the pinion flange at the same place....get some paint and mark things.

I just removed the pinion nut, counted number of turns.

Dug old seal out with a pliers and punch (carefull...next time I'll use a punch and a slide hammer with a screw) and tapped new one in with an empty can of peas or soup can....whatever fits just inside the seal. YOu can also just tap it in around the sides. Use some grease on the inside spring to hold it from flying away while all this tapping is going on...if it goes away or drops off you have to start from scratch with a new seal because it's almost impossible to replace the spring without pulling the seal and tough to pull the seal without damage.

If the pinion flange is severely grooved (your fingernail catches on it) it generally has to be replaced although you can 'try' it with the new seal and see how much/whether it leaks. If it leaks, you're not out much more than the time to drop the driveshaft at the rear and reinstall the pinion flange.

Yes, there is a technique involving inch-lb rotating force at the pinion bearing to set preload, but if you're careful you'll get adequate (too much is the problem) preload by just counting revs/flats of the pinion nut when you reinstall the flange.