Sorry, I dunno is the honest answer, I got that far and solved my issues ..... Personally I'd have a really good look at the linkages 'cos on mine it looked like it was working but a bar at the top wasn't moving quite as it should. Fixed it there and it worked.
I'd also, as you've suggested, undo stuff until I got to a part that was broken.
As an aside, are you sure you're changing gear properly? Not being funny, but my mate drove my Disco and said he also couldn't get from Hi to Lo ... but he wasn't moving the lever far enough!. It's sometimes also worth feathering the clutch as you do it, get it on the point of biting as you change.