My 98 ext cab has weak rear leafs and bottoms out with an empty trailer or with 300 pounds in the bed. What is best add a leaf or clamp on stiffener I call em to keep or improve ride and add payload over 500 ?

 

Take your truck to a spring shop and have them make up some replacement leaves. Tell the shop how much weight you want to carry and how much lift you want. They may need the truck to take measurements but should be able to make some right there.

I got some made for mine and it worked out good. I kept the 1st spring attached to the frame and replaced the remaining leaves in the pack.

 

These trucks have notoriously soft suspension. They were built for offroading, not for work. They'll work as work trucks because they're built well and built to last, but it wasn't the intended use, so it takes some modifications. I pull a 17' camper trailer with my 98, probably 300-350 pounds tongue weight plus my topper and a lot of gear in the back of my truck. I have both aftermarket HD leaf springs (I replaced the entire leaf pack) and air bags in the back. That keeps my truck level. But it doesn't do anything to increase the payload. Keep that in mind.