It is never recommended in any 4wd vehicle, UTV or truck (does not pertain to all-wheel drive) to run in 4wd when not necessary, at excessive speeds.. or on hard surfaces. With these, even though all is still spinning while moving in or out of 4wd and the only difference is the axles are coupled to the carrier, I would avoid speeds over say.. 30mph in 4wd just because of stresses on components...replacement costs are over $900 usd after all....and I've seen a few broken cases. In 4wd with the diff-loc on there is an RPM limiter for safety. Whenever that comes in to play might be a good reference point to be out of 4wd too. Just thinking out loud.