It's probably right at the point where the TC is deciding whether to remain unlocked or locked?? Since it's all computer controlled and the computer does not know what the exact driving conditions are at that moment in time, it's possible that it's right on the edge of doing either?
Is it a 4, 5 or 6 speed trans?
Did your brother drive it during the test drive or was it the tech?
The vibration is probably an out of balance wheel up front? Or the OEM tire being out of round?
Hell, it's a brand new rig and it's just working out it's new car bugs.....
Just my thoughts on it.... Having never driven one of the newer F150's I'm only yakking out of my a$$ really!
Mitch