Is that back end sagging as much as it looks like in this photo?
If so, I highly recommend a set of Timbrens.
They're a suspension enhancement system that are kind of like airbags, except there is no compressed air.
They're just a cone of rubber that progressively resists more as more weight is put on it.
They're pretty cheap, about $175/pair, so if they don't help you're not out much $$. But they will
In most cases, you don't even need any special tools or drilling to install it - just jack up the truck and take the tire off, remove the existing bump stops, and put these ones in.
With my 2013 GMC Sierra, there were no nuts/bolts, it was a friction fit. I levered the bump stops out, put the Timbrens in, put a piece of 2x4 in there, put the tire back on, and let the truck down.
The 2x4 being in there smashed the Timbren down into the holder really well. Then I jacked it up, took the piece of 2x4 out, and it was all good.
Here is the site to find the right kit for your truck: Suspension Enhancement System Lookup - Timbren Industries
I looked up a 2009 Toyota Tacoma and the kit was TORTUN4. Looked it up on Amazon, and it has some pretty good reviews: http://www.amazon.com/Timbren-TORTUN4-Suspension-Enhancement-System/dp/B001TNBTJI/