4 days and no answer? I'm always interested in what someone else finds, especially when I've had the same/similar problems. New here myself and I found answers I've tried to find for years. I have a back yard full of mid-late '80s DL & GLs with tranny problems ... lost high in drive but would pull off in low and shift to drive 2 with no problem. I have started pulling the governors on the trannies that are out and found them to "all" be "apple-cored". IF one only knows where to look, he can save a lot of wasted time and money. From experience, I knew the answer had to be simple and we tend to overlook the simple things, right in the beginning. THIS is why the "speedo" working/not working is important. IF the "speedo" is working ... it shows that the tranny is working, it just isn't being transmitted to the wheels. Given that the wheels are free to rotate and the "speedo" is turning, with it in gear and at a high idle, place it in Drive 2. IF neither wheel is moving but, they will easily move by hand ... from under the car, check both axles for rotation. No Rotation? Hold the palm of your hand, lightly, on the underside of each inner joint, at the same time. You are feeling to see if there is any apparent movement inside the differential and, if so, which side is more sensative than the other. You can also get a cheap stethiscope at Auto Zone, etc. and "listen" to different areas around the differential. Believe me, crazy things happen to cars, just ask any mechanic. Also, drain the differential oil and see if it has a shiney metallic look. In '84, I had a customer's Audi that ended up having an inner CV Joint that split in to, from a hair line crack adjacent to the splined section. Once ... in over 40+ years of turning wrenchs and owning a shop. Just because nobody's seen it, doesn't mean it won't/can't happen.
Excuse the length and good luck.