I also had a noisy transmission with my '86. It also only got quiet when I pushed the clutch in AND in 4th gear. Last year I pulled the engine myself and rebuilt the engine AND did a full clutch kit because I was hoping my noise would go away after changing the throw out bearing...no such luck, still noisy in all gears but 4th and when sitting idling. I had my trans pulled and rebuilt at a reputable shop and yes, my noise was coming from the big input bearing in the transmission. The guy gave me the old bearing, and yes, spinning it and playing with it you can tell it had been noisy and wearing for a long time...its pretty sloppy. Let me know if you have any other questions but it sounds like our trucks are similar enough I'm afraid you have the same problem I had. I'm in N Colorado if you want to come look at this sloppy old bearing!!
Bill