An alert has popped up on my dash saying "Service 4wd" the 4wd will not engage and there are no leads as to what this issue could be stemming from. Any help would be great Thanks.

Bad vibration at highway speed intermittently. Starts at 75mph and peaks at 80. Factory tires, rotate every oil change. When it slows below 70 vibration stops. Doesn't happen every time and is more pronounced in 2wd vs AWD/4WD. Had tires balanced last rotation and still happens. It feels like its coming from the font passenger side but is not visible from another car following next to me.