I ordered new factory sensors through Amazon at a fraction of the price from the dealer. I checked them with a ohm meter and they both had a similiar response to the rear sensor so it looked like the front sensor was shorted out. I replaced the front sensor and it has a crack on the face of it. After replacing the sensor everything returned to normal and I was able to read the front/rear driveshaft speeds with the OBD-II reader.
I went ahead and hooked up the JYEtech oscilloscope to one of the speed sensors and also checked the duty cycle being sent to the electromagnetic clutch. The speed sensor outputs a nice 5 volt square wave signal proportional to the speed. It outputs about 840 Hz at 40mph and would be simple to measure with an Arduino. It outputs both front/rear driveshaft speeds whether it is in 4wd or 2wd.
The electromagnetic clutch signal from the GEM runs to a solid state relay and is inversely proportion to the output meaning it is high most of the time under no load and goes low more of the time when wheel slippage occurs.
Following are a few pictures of the ocsilloscope and the sensor.
Hall Effect sensor at 40 mph and photo of sensor. GEM with probe attached.
Duty cycle at idle, duty cycle in 1st gear gunning engine on packed snow. Duty cycle with bad speed sensor.
Once I get some time I might write a script for the Arduino to monitor and record the driveshaft speeds and the clutch duty cycle. I assume I would need to use a MegaArduino for the extra interrupts to measure three frequencies.