rear end sag with load, and transmission slows truck down

Just replaced my gas hog tundra, for a 05 Tacoma double cab for my work truck.

With the contractor's cap and my tools the rear end is sagging a little (guessing around 500lbs), the truck has 116k on it.
I am wondering which is the most reasonable way to correct this?
I was thinking an ad a leaf spring or maybe try to get a hold of some tsb leafs. Also when I towed my boat it looked like it was riding a wheelie, not a big concern - probably gonna dock my boat this year and won't do much towing.

Another concern is, when I let my foot of the gas, the transmission immediately slows the truck down. I seem to have to keep my foot on the gas even when going down hills.

Dose anyone know of any transmission programming that can adjust this?
I think this is going to hurt my gas mileage.

 

Firestone ride right airbags would help the leaf springs.

 

The slowing down you are referring to is most likely engine braking and a normal function of the truck

 

There are a bunch of guys including myself that run caps, racks and tools, I have a 2 inch AAL and Firestone Ride Rite airbags, these leaf springs are pretty weak. If you have the money, replace the leafs altogether but if not what I did works very well.

Welcome to TW

 

I'm having a local spring shop add a fourth leaf to my spring packs this coming Friday. This is NOT simply one of those add-a-leaf jobs, but a complete disassembly of the spring packs and adding a near-full-length leaf between the current 2nd leaf and the overload leaf. This is a very common fix for stock older Tacomas where the original 3-leaf packs have weakened with age & use, causing a saggy butt and bottoming out. Cost = $420.00. No need for new rear shocks either. In fact, new shocks specifically NOT recommended until I drive the truck with the new springs and see how it drives & rides.