It looks like you got your answer, but I thought I'd chime in anyway...
I've seen threads on this before, and it seems like the axle-to-bed alignment is not perfect on any of our trucks. Some Rangers seem to be off more than others.
Like it has been mentioned, your leaf packs can throw the axle alignment off, but it's generally how the bed is centered on the frame. You might check the gap between the portion of the bumper that extends under the tailgate, and the edges of the bed sides where they wrap around the back corner. Then again, your bumper alignment could be off too.
A shadetree mechanic's test for crabbing (or a bent frame after a collision, for that matter) would be to drive your truck through a puddle of water and then get out and look at the tracks left on the dry pavement. That will show you if the rear axle is following the front tires, or if it is out of alignment, relative to the frame.
2003 Ford Ranger | FX4 Level II | 5-Speed Manual
A few minor mods... nothing cool enough to mention.