According to the manual - 55mph (88kph).
I have shifted mine from 2wd - 4wd at 120kph, and it shifted silently....but then again I backed off slightly during the shift.....:4-dontknow:.....if you do it above the recommendation, you are taking a risk.
I shift in/out at highway speeds all the times, but the reccommended is as Brad said ~90km/hr. But keep in mind, if you're shifting into 4WD at highway speeds, then you should probably be reducing your speed to allow for more control and shorter braking distance. 4x4 does not help you stop faster.