Shimano 9 speed road hub compatible with XT 11 speed cassette I am looking to upgrade my drivetrain, essentially moving from road (Ultegra and 105) to XT mountain.  I'd like to avoid replacing my rear wheel if possible though.

I've read conflicting things about whether the hub spacing between road and mountain is compatible.  Can anyone answer definitively?
Changing from a 9 speed road to 11 speed mountain raises many, many questions, but answering your specific query:

An 11 speed mountain cassette will fit on a '9 speed' hub. The freehub body width used is the same for mountain 11 speed and below and road 10 speed and below. Road 11 speed cassettes generally requires a slightly wider freehub body. The mountain 11 speed cassette will fit because the larger sprockets can overhang the hub and spokes a little.