Lewis,
I've been watching this thread with interest. I don't have XTR's but am using XT's on a tandem - they do the job pretty well, but we do have a mechanical disc on the back as back-up. That judder does sound severe and there must be a cause for it that get's around the need to clean your rims with two different kinds of cleaner on a regular basis - what happens on a long ride?!
I wonder if this would help you get to the bottom of the judder - it is essentially caused by clearance/play between the various components and/or hitting a resonant natural frequency:-
Try deliberately tightening the headset just a bit tighter than normal - so that you just start to feel a very small amount of resistence to turning the bars. Obviously don't overdo it! and reset it again afterwards. But see if it eliminates the judder. It may be that you need to go just a little bit tighter than you are used to.
Have you tried the suggested 'front brake on and check for play' test (easier with 3 hands!). I am not sure about the history of your machine, but could it be something like the crown race being a slighly sloppy fit on the fork steerer, which no amount of tightening will take out - you would feel this if you put a finger across the bottom headset cups and the fork - a thin layer of tape around the base of the steerer tube is a temporary way to eliminate this if it is the case. There are slightly different standards for crown race internal diameters which can cause this. I have had to both apply tape and open out crown races to make things work at different times in the past.
If you can definitely eliminate all traces of play in the forks and there is still a problem, then try putting some heavy grease on the v-brake bosses - keep it away from blocks and rims - it could be the the fit of the brakes onto the bosses is a bit loose, and the grease could damp it out enough to tell you if this is where the problem lies. (I realise the moving parts dont bear directly on the bosses).
Try jamming the brake on and rocking the bike back and forth and looking very carefully at the brakes to see if you can spot any signs of slackness/play. There will always be some, but it might give you some clues as to what is causing the judder.
Another thought - the XT pads are quite long and I assume the XTR ones are as well. Do you have any shorter shoes/pads you could try?
Don't know if any of this helps but might give you something useful.
Cheers,
David
