ive had a really strange vibration past couple of days..... something i havent read about on here before...... when i turn the fan up all the way (whether on heat, ac, defog, w/e) theres a strong vibration throughout the truck, especially in the brake pedals....... any thoughts?

 

Um ... blower motor wheel (squirl cage fan) loose on the shaft ... maybe?

 

That would be my guess. Check the fan to see if the bearings that hold the fan shaft in place are worn. Something else is to see if something has gotten into the fan and stuck to it and made it off balance......