Yep - knobblies don't help with high speed stability. As above, knobblies seem to be quite pressure dependent with regard to on-road handling.

It is also worth checking the steering head bearings for excessive play, notchiness, or over-tightness. They can play havoc with handling and its quite a common XT failing as they never got a full dose of grease at the factory.

Same at the rear - check the swingarm pivot and shock linkage for excesssive play. Slop here can have the same effect and cause wobbly, odd-feeling steering.

Try adding more preload at the rear, something as simple as that can have quite dramatic effects. While you're at it, it's also worth checking the suspension sag front and rear is even and adjusted within reasonable limits - work on 30% of total travel for rider or dynamic sag (ie: with you on board).

It could be loads of other thing too... indeed high screens, mudguards and luggage/top boxes can all cause vague steering at speed. Also think about you riding position; too far back, too upright, grip too tight on the bars? Try different positions.

Main thing is, try an adjust only one thing at a time. Do too much all at once and you'll never isolate the cause.

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