not to throw a whole new problem into the mix, but I had a similar sound in my 85. Turned out the rear bearing on the Tranny was going, and as I let off the gas it would allow the mainshaft to shift just enough to make a wierd noise. It eventually popped a ball from the bearing which meandered thru the tranny and ate 4 of the 6 gears.
Drop the rear d-shaft, and see how much play you have at the rear out-put shaft. Should move only minimally if at all. Also, while your there, look for any signs of leaks, which would suggest a bad seal, which in turn would suggest you may be low on 90w, and just need to top it off.
You could also play around a bit and take the rear d-shaft out all together, drive only on the front for a short distance and see if it still makes the same noise. If not, I'd look at my rear ring and pinion (check gear oil level there too).
gotta play with it some to try and isolate the noise.
Erik
