My 2004 F150 4 door bumps at least once and sometimes twice when I start moving forward. I feels and sounds like the location is in the rear axle. It is very much like the feel when a u-joint is going bad but the drive train is tight. It does not do this when the temperture is less than about 40 degrees. Anyone ever had this problem and if so how did you correct it?

 

You state that the drive line is tight but have you put the truck on a lift and checked it.
By taking the pressure off of the wheels, axle and drive shaft you can shake and turn things to find out if there is play that is not apparent when sitting on the ground