This doesn't address the weight & bouncing issue, but sometimes it's a bunch
of little things which, added together, make a big difference. Recently,
someone posted a question here about the tendency of these trucks to wander
a bit at highway speeds. If I recall correctly, the solution was to make
sure that during wheel alignment, the technician adjusted as close as
possible to dead center of the allowable range, instead of simply "within
the range". In other words, if the range was 1 through 10, then 2 was not
good enough - it had to be 5-ish.

As far as tires, I have Nokian WR tires on mine. I inflate to 35 lbs cold
pressure. No bouncing around, unless I'm on a road that's hideous.

A set of Michelin M-S tires made a big difference here, along with retail
Bilsteins.
The OEM tires are crap, very soft in the sidewalls.

1)  Air up the tires.
2)  Align to the exact mid-point for camber and toe-in spec, and the
max for caster.
3)  Better tires.
4)  Hellwig rear antisway bar
http://www.hellwigproducts.com/antiswaybars/jeep_toyota.html

Ken

 Someone mentioned a good alignment, and making sure that the
settings are biased toward the good handling side - you do need a
little toe-in and caster to keep it stable.  If they have it set too
close to zero, it can be "within specs" and still be the problem

 Good idea, and it will also help with the ride of the vehicle - the
springs are set to handle a full load, totally empty is too light and
it rides like a brick.

 Though I wouldn't use concrete blocks - bagged sand is better if you
just want portable weight.  Plus, it's great to have and spread as a
traction aid if you get stuck on a patch of ice.

 Bagged beach pebbles are good if you don't want to chance loose sand
getting into things, or the sharp corners of crushed gravel wrecking
the paint.

 Or just put a shell or tonneau cover on the bed, and the natural
accumulation of "stuff" inside (ice chest, golf clubs, bowling ball,
toolbox...) will handle your ballast weight needs nicely.

   --<< Bruce >>--

Off the original subject, but if you eventually begin shopping for tonneau
covers, take a look at this:
www.versacover.com

I got one two years ago and I love it. My only problem is that I lost a
little storage height within the bed, because of the reinforcing ribs of the
cover. So some stuff doesn't fit. The cover itself has been fine, though. No
leakage, very solid. Around $600 if I recall.