Hey man. Im no mechanic, and don't pretend to be. But when i bought my truck a year ago, i had a similar vibration from front end. Slowly got worse over the period of a few days. It was winter so i had been using 4x alot. Took it to nissan and they ended up replacing the Propeller Shaft (i believe it's AKA front drive shaft). I read on here just the other day from someone else that our front u joints are not very good as they are a sealed unit, and not serviceable. Hope u get 'er figured out.