The front tires on my 06 150XLT are wearing on the inside. I took it to the alignment shop and was told it was in tolerences. It drives and stops straight. Any ideas?

 

Rotate your tires more frequently and make sure you are running at the operating pressure listed in the manual or on the sticker on the driver's side door panel.

 

There are three possibilities that I can think of at the moment.
One is if you excessive toe out the tires will show feather edging and wear on the inside.
Another is excessive negative camber on both wheels.
Also a good check for worn ball joints.
Some "alignment" shops don't carry the offset spacers to do a good alignment on some Fords. You might consider getting it checked by a Ford dealer. They will have the necessary parts.