Might check to see if a brake pad is broken.

 

Check the caliper pins .
They get gummed up and cause all kinds of problems.
You should be greasing the caliper pins every 1000 miles.
If you are not keeping the caliper pins greased the calipers will drag and cause the pads on the caliper to heat up and lose braking power.
If the truck pulls left it's usually the right caliper that is sticking.
Be sure to install new rubber bushing's everytime the calipers are removed.
Also check out the "Member Photo's" section for some great how to pics and tricks.

Happy Braking

SFB

 

Check out my you tube video.
http://www.youtube.com/watch?v=RGHd5-j2TQk
It shows how your wheel should spin.
If your wheel doesn't spin free your calipers are dragging.
Also check out my Member photo's for more great tips and tricks.
Remember to grease your caliper pins every 1000 miles.

SFB

 

Check wheel bearings too, mine was doing the same thing. Ended up beeing wheel bearing. Then the truck did it again a few weeks later and rotated the tires and problem was gone.