For most trail riding where your moving along at 20 mph or more then high is fine. If moving along at slow speed or thru mud or serious hill climbing then low might be a better choice. Key reason is keeping the drive belt cool. When running low engine rpm is higher keeping belt fan at a higher speed to cool the belt. In very rough trails or going thru heavy mud the cvt shifts frequently causing the belt to overheat (possibly) so this is where low would be an advantage. You manual has a breakin period, no wide open for at least 100 miles, vary your speed frequently, check your oil frequently. As far as 2wd into 4wd on the fly yes you can but be sure to back off the throttle when doing so, do NOT shift on fly while accelerating.