For context I am set up with a more overland build with 33x12.5s and I don’t really intend to do any crazy rock crawling. I have good pinion angle and have no rear vibrations, I sit right around 4-5 degrees as per Adam’s DS recommendation. I currently have IRO 3.5” leafs with their 1.5” boomerang shackles in the rear. I have a rear bumper with tire carrier and I am often loading the rear with lots of weight: bikes and bike rack, roof top tent, gear, fridge, etc. and it seems to be sagging a bit. I picked up some used RE 3.5” leafs. Is it worth switching them out? Should I keep the IROs and get shackle relocation brackets to add more lift in the rear? Should I swap to the used RE springs AND get SRBs? Has anyone had experience with IRO springs and lots of weight in the rear? Will I need axle shims to fix angles?

 

According to the internet, the RE are a bit stiffer than the IRO, 190 vs 170 lb/in, so they would be a good upgrade. With the sag seemingly occuring solely in the main leaf between the eyelets and the ends of the 2nd leaf, on my next go-around, I'm going cut the eyelets off an old leaf and insert it into the new pack, flipped over so that the sag now helps hold it tight to the main leaf. Not sure what the final spring rate will end up being, but it's not a new idea...

If the unladen ride height doesn't change, you won't need to shim the rear. Change it enough and you will. I can't imagine a 1/2" or so would necessitate shims, but I guess it depends where you're starting off.

Bump stops are key to preventing sag and keeping tires off the sheet metal. I modified DPG's concept, just using a steel plate on top of the Lift Spring Plate and just an extended-height bump stop, no fancy bolt-on spacers like DPG uses.

 

Alcan Springs, I had 4.5" inch lift built, the shop (great service) had me weigh my rig empty, empty just the rear, and loaded as I intended to drive it daily. They built me a high quality set of leafs that ride beautifully (as long as I have my gear in it, its like a buckboard empty LOL). I as well built it for overlanding and fully loaded for a 4 day excursion it didn't squat and it rode great. They are custom built and there is lead time.