I was in the West of Ireland a few years ago and stumbled across a meeting of the Zundapp/BMW owners club for Eire. These were the original WWII style bike and boasted diffs to the Right hand sided sidecars. I've got some pictures somewhere I will post one day. Certainly there was a lot more there than the simple dog clutch on 'modern' Ural/Dniepers. I've not yet seen an older style Russian made bike with a diff but think it would be best without anyway, as it would be for off-road use only in any event and of no real advantage on the road. Where I think they really score is in having a sidecar brake.
I was wondering if buying a 2WD Ural in France and riding it back, before re-registering for UK road use, might be an alternative to the Tricycle registration route, which is a bit dodgy IMHO. As for the swapping of frame numbers.............
Richard