Hey...fun post...thought I'd add my $0.02 in...
Found a pic in Car and Driver (I know...it's not a truck, per se, but for purposes of discussion and adding to the knowledge base).
Anyways, this pic is of the rear of the #19 Dodge from Evernham Racing
.
It shows the fabricated trailing arms are not "boxed" in a traditional sense. They are reinforced with plate steel, at least on the bottoms. Can't see the tops.
I think you can also see the Panhard mount on the right arm, previously mentioned by stl0066, if you squint reeealllyy hard.
Anyways, it is one clean machine.