Recommend checking the plastic pins that hold the bed rail caps in place

These pins are inserted from inside the bed.

I found two of them laying in the bed, so I stopped by the parts counter at my local Toyota dealership to buy replacements for all four. It turns out the other two were very loose and ready to fall out.

I highlighted the part #90467-07188

 

Was there any indication they were loose like the caps wiggle or something?

 

The caps seemed to have a little bit of movement fore and aft, but it wasn't significant. I just thought they were worn from being 15 years old. It wasn't until I saw the old pins sitting in the bed that I started looking for where they came from. The caps felt tight again after I replaced the pins. It's back to normal again with no fore & aft play.