Like the other guy said, you want springs not shocks. I tried to get away with my sleeper platform without heavier duty springs for a while and it sucked. Bottomed out all the time. Mine is always full of gear too so it ends up being ~300lbs of weight in the bed on what were, as far as I could tell, the original springs.
Unfortunately I couldn't find any heavier duty springs that wouldn't also lift the truck and require using non-stock shocks. I read in some threads where people said they ran 4600s and OME 891s but people always chimed in to say that was a bad idea so YMMV.
I went with OME 881s in the front and 891s in the rear with 5100s all around. Here are some not so great pictures, the wide angle lens makes it look more dramatic than it is. I got it all here and paid the extra to assemble the front shocks which makes it all easy.
If you're really dead set on not lifting and want to go the cheap and easy route lots of people run these airbags and report great things. They need an air source but you can literally use a bike pump.
Nah it's super easy. I used a sawzall to cut the rings off the bumpstop. Cut 1 at a time or you'll have a big chuck of rubber that you can't remove. (ask me how I know)
Push the airline up through the rubber bumper from the bottom and run it where you like. I run mine through frame and out the ends. I use foil back sound deadener to wrap around airline near the exhaust and where it goes through the frame. There is a hole in each bumper where the side pieces meet the center. I put my valve stems there. I prefer to have each airbag independent so you can adjust the pressure if you have an unequal load in the cargo area.
Just collapse the airbag by squeezing all the air out, and push it in with the air line connection going up.
Put the barbs on the airline, connect to the airbag, put the valve stems on, and you are good to go. Should take an hour tops.
I have had the kit on my 1999 since 2014. I run 5psi when unloaded. Run up to 50psi when loaded down with a trailer or gear in the cargo area, or heck, even if you have your whole family in there.
https://www.amazon.com/LIFT-60743-1000-Air-Spring/dp/B001F15Z08 - This is the kit I used. I paid 83.85 on Jun 26, looks like the price has gone up by 10 bucks.
https://camelcamelcamel.com/product/B001F15Z08?context=search - This site lets you flag Amazon for price alerts and tracks pricing history. Looks like the price has been up and down as low as 72 bucks in March!