2008 6.4L Powerstroke, DRW

My truck just started acting up recently. I am getting a vibration/noise somewhat frequently, when towing a 16K load.

When towing, the sound will come and go. The best I can describe, it sounds/feels like I am driving on a rumble strip on the side of the highway that wakes you back up if you drift out of your lane.

It will last anywhere from couple of seconds to 30 plus seconds and happen multiple times when in tow, especially uphill.

Any thoughts on what this might be?

PS. Other things to note. Fuel economy has always been poor on the truck, but seems exceptionally poor at the moment. This could be from heavy towing though. It is also regenning about 50% of the time that I tow, and I tow often. On long trips (4hrs) towing, it is not uncommon for it to regen several times during the trip.