Wait till you get buzzed by a B-52...That would really rattle your truck... :-)
It was probably a unit training in a non-traditional enviroment. I've done it myself...nothing to get worried about.
Not worried a bit. Like you said, very cool. Just curious. I grew up in the SG Valley and returned about 4 years ago and have never seen this much military activity in the area.
I have been buzzed by an F-4 skimming treetops. I was a lad at the time riding with my mom in central Texas. We were driving along a winding Hill Country road when this F-4 came screeming through the valley just a few feet above the trees. Mom nearly jimped out of her skin (it passed about 50 feet ahead of us). I, being a 10-year-old aviation fanatic, was elated beyond belief.