Mike, I have a '91 745t that exhibited the same problems you describe. Besides the sensor mounted back of the differential, there is another common problem with our 740 speedos. There is a printed circuit board behind the speedo which sometimes does not age gracefully. I'm told there are some capacitors mounted on the board which often fail with age. These can be repaired by a shop which specializes in this sort of work. I found a speedo shop via Google in Austin TX which does this work, but have lost that info. If you can find someone who repairs solid state electronics they perhaps can isolate and repair the failed components.

I don't recommend replacing with a unit from the boneyard because they are just as likely to fail if not already broken. By the way, before I could pull my unit to send to Austin, it started working again. I expect it will fail again once cold weather sets in.