I'll agree with Snoman (to a point) about suspecting the e-brake.  Simple
test...  with the truck loaded, jack up the rear (just one wheel if you have
an open diff - if you have a limited slip, you'll need to raise both rear
tires), and with the truck in neutral, see if the rear wheels will turn
freely.

Or, you can simply disconnect the e-brake cable at the adjuster (along the
driver's side frame rail, about half-way between the cab and the rear axle).
Loosen the adjuster nut until you can disconnect the cable.  Tie it up so it
doesn't drag, and test-drive the truck.  You can also crawl under the truck
and take a good look at the e-brake cable, then have a few friends jump in
the bed, and see if the cable tightens up or is otherwise fouled.

As for the comments about getting a bigger truck (no doubt with a big-block
gas engine, and super-low gearing in the rear axle), well...  ignore that
part. An '02 Dak is more than capable of hauling 400lbs. of cargo.

Another thought...  does your truck have a load-sensing proportioning valve?
Follow the flexible brake hose from the rear axle up to the frame rail.  If
there's a valve mounted there with an arm, and a rod that runs down to the
axle, that's a proportioning valve.  If you do have one, I'd also throw some
suspicion that way (and, in fact, that's the more likely culprit than an
e-brake cable getting hung up).  That valve's job is to sense the amount of
weight in the bed, and allow progressively more brake pressure to the rear
brakes as the load increases.  It's possible that this valve is holding
residual pressure in the system, and causing your rear brakes to drag.

Of course, if you don't have the valve described above, ignore the comment.