I don't work for FedEx, but all these freight co's are the same. Linehaul you go from point A to point B, at FedEx you work the dock for a while the go back to point A. Dock is on the clock, driving is mileage pay.
P&D is point A to B, C, D.......well you get the point. It's paid by the hour, so you've got management wanting you to acount for any delays etc. If you don't know your area well, it can be a really long day. It also helps to be more of a people person in the city, helps ya shmooze customers to help ya out if you're late, need freight shuffled around on your trailer etc.
I've worked at a few carriers, and I've always prefered Linehaul myself. After almost 10 years in sales/customer service, I like not having to deal with people.