Need help identifying rusted part under truck bed

[Cynthia Grant]

2003 Trac XT with 235,000 miles (4 WD)
Can anyone please tell me what this part is that has the rusted HOLES in it? This view is into the driver's side REAR wheel well, just below the fuel filler line. Obviously, my ST has a lot of rust, but the engine is still running like a champ. I just would like to know what part those holes are in and whether this is an urgent issue. Due to its age, this beloved truck is my backup now (not driven daily), but she's given me a lot of good miles and I'm hoping to keep her going a while longer.

Thanks!
Cindy G

 

[Todd Z]

That looks like part of the frame, and if it is, that truck is done and unsafe to drive... motor good or not.