I posted a few weeks ago that my 740 Turbo wagon (90) was grinding in 1st gear, couldn't hear it in other gears. I noticed driving up hills in San Francisco. Well I went kayaking yesterday and had an opportunity to back up a boat ramp, and was surprised to hear the noise in reverse. I'm wondering if this could mean that my transmission could be off the hook, and I should be looking more closing at bearings between the transmission and the rear wheels that are heavily loaded when I go up hills. Any thoughts on how I can locate my problem more precisely without taking too many things apart?

Could be your drive shaft universal joint where it attaches to the rear axle. They usually start going bad by making a grinding sound when your first start up or put a load on the drivetrain. Set your parking brake and put the car in neutral. Wiggle the drivshaft and check for any loosness in the joint.