Mine's doing the same thing with a bonus. 13sp in a 99 9400i that bangs into gears and will also pop out of 12th and 10th if you let off the throttle with the jake on. Think this would fix that or is it another issue?
The popping out of gear would be a different issue. Your top 2 gears and the sliding clutch for those 2 gears are helix cut with a pitch or angle to aid them in staying in gear under load. Reverse the load by using the jake and they come out of gear easier than normal because of the pitch on them. I would look at the shift fork and sliding clutch for excessive wear. This could cause the sliding clutch to back out of the gear when load is reversed.
To sum it up, a helix kit, the shift fork and renew the shift detents and springs in the shift cover should take care of the popping out of gear.
Correct me if I'm wrong, but I think you're describing going from low to high range and back into the low hole position for sixth. If so, that's always been a hard hole to hit because of it's position in the shift pattern and sixth gear is commonly skipped because of this fact.
One other thing, aside from it lurching and surging all over the place every time I take off, the top two gears (or 4, or the two on the right) seem to have some kind of progressive shift garbage that won't let you into or out of gear most of the time. Is there a part you can yank out to get rid of that? Or is it just something else worn out?