OK, I brought home a high-end HP DMM from the lab at work and took the measurements. The diode readings were all ~2.01V in the FB direction and "OL" in the RB direction, so all seems well there. The resistance readings were all 6.7-6.9 Mohms in the FB direction and 181-183 Kohms in the RB direction. So I'm not getting the ~4 Mohm readings in the RB direction that your post suggests, but I suspect that's just a difference in the how the different DMMs are measuring a circuit that isn't purely resistive. The fact that all four readings are essentially the same tells me that the 4 wheel sensors are probably OK.
I didn't do any of the other tests because none of the 4 wheel sensor readings differed from the others. So I'm inclined to conclude that my ABS module needs to be repaired or replaced.
Two questions - if I send the ABS controller in for refurb, can I still drive the car without it? And if so, do I need to cover the exposed portion of the ABS system with anything to protect it from the elements?