Heya 'roadie', many thanks for the reply. I am 'eshanon's' Dad.
When this problem originally cropped up, I did some research and ended up replacing the encoder motor.
Generally, a bad encoder motor will light up the "Service 4WD" lamp and you can read a code with a decent (not low end) scan tool. The essence of an encoder motor is that it has a position feedback sensor to report back to the TCCM that it successfully moved to its commanded position. If it fails to move by virtue of a dead motor, jammed planetary reduction gears, or a jammed transfer case internals, the Transfer Case Control Module will notice and complain. That's why I wouldn't immediately suspect the encoder motor unless you also had a code thrown.
I am an old-school mechanic. I had two full service stations operating back in the 70's. I consider myself very qualified to work on older vehicles, but I am not up-to-date on all this computer controlled stuff. I used to do a lot of high-performance work for the locals, mostly on muscle cars (like the ones you see on Mecum or Barret-Jackson auctions). Man, those were some fun times.....
Very, very cool. I'm an electrical engineer, with a 38 year career in designing and maintaining semiconductor test systems. Amateur working on cars, but when I bought the Roadiemobile, I also bought the factory shop manual and spent over a month of nights and weekends reading it cover to cover. So the theory of operation for the various systems is now in my head and I can't seem to get rid of it.
And some things weren't totally detailed in the manual, but I know how the designers must have thought, and some things like HVAC actuators, failed on my truck and I was frugal enough to want to understand and fix the dead item instead of just buying new ones. So a lot of my posts, especially years ago on the other site we all met at, go deep into the theory of operation of esoteric systems like the HVAC and 4WD.
Anyway, I am most curious as to your reasoning on a potential problem with the front diff. The vehicle does have limited slip rear diff. But on the front, how could the front diff possibly be producing the symptoms as described?
A front diff failure is usually accompanied by grinding noises and broken bits of gears, but if the noises are misinterpreted, the intermittent jamming can be thought of as binding. But again, a few minutes on a sand or gravel surface with an outside observer should reveal if it's a front end or back end issue.
In the rear, I'm not totally familiar with the options on the Saab, the rarest of the six marques using the GMT360 platform. But except for the Trailblazer SS, I didn't think any others came with the G86 LSD. A bit of Googling says that's true. All the other five marques had the G80 automatic locker as an option, but the Saab only had an LSD, on both the 5.3 and the 6.0 L V8s. Has nobody pulled the rear diff cover and inspected the clutches?