Well I know something no one's mentioned here.

Excluding any possible issues with steering head bearings, I know what is the major culprit for front wheel wobble on the XT bikes, because I've heard it straight from the chief designer of Avon Distanzia tyres, and the issue is: excessive tyre pressure + knobbly to semi knobbly tyres do no play nice together.

So the fix is simple reduce tyre pressure to 28psi and go for a ride, if no wobble, then try to up it to 29, no wobble the go up another psi, soon as the wobble returns then you know you've gone too far so back it down again.

As I said if there is nothing wrong with other possible issues, then the tyre pressure IS the problem.

Oh and please make sure the tyre beads are fitted evenly around the rims without bumps on the tyre walls.

Try it, and report back please.