Aloha Windrose,
Doesn't look like you're getting much satisfaction on your transmission issue. I tend to agree with steveg that it might be time for a teardown. At a minimum you might want to drain your transmission oil, and check for bits of metal, which may at least point to/confirm a larger problem. Years ago, when I took my BT7 transmission apart I found that one of the gear selector forks had a missing "ear". It still shifted OK but wasn't entirely smooth. I'm not suggesting that this is your problem, but from the sound of things, one of the gears is not fully engaged. If memory serves me, the selector forks are pinned/bolted to the selector shaft. Perhaps this bolt or the selector shaft itself is the culprit. Again, try checking the oil first......
Maybe this will inspire someone with a stronger technical background to chime in.....sorry I can't be more helpful.
DB