It's a strange issue, but I'm going to solve it
As suggested I've checked the seating of the bearings on the gear diff. They were tight to put on, but are properly seated against the edges of the diff. Also, with only the diff in the tightened casing, it still binds.
I think I've just got the wrong combination of a wide diff and a tight gear casing, resulting in a tight layshaft at first and now a tight gearbox with a gear diff. The layshaft was solved by 0.2mm shimming, so the gear casing could be as little as 0.15mm too narrow. The wide diff is 0.4mm wider than the ball diff - quite considerable, but keeping in mind the stuff is all injection moulded things like this can happen.
I'd like to see Durango respond soon to the email I've sent them. In the meantime I'll have to take matters into my own hands. The gear casing would be the cheaper choice to mod if anything goes wrong, and I have dremeled out bearing seatings for my TRF to fit a gear diff... but it's a risky job. I'd much rather take some material from the gear diff's side plates as it's more accessible
Stay tuned!
Edit: Ok, it worked... but I feel really stupid! After dremeling 0.4-ish mm off it was rotating smoothly. I cleaned the diff and the housing, but before re-assembling the whole thing I decided to check on the gaskets to make sure the brake cleaner hadn't reached them. It was all good, so I closed it again, but this around I have overtightened two screws!
I can now feel it's not tight in one part, then in a small part of the rotation you can feel teeth rubbing each other, as if a motor is cogging. I guess I'll be making Durango happy with another sold item, though not the way I was hoping to...
