without that sand weight i believe they are just flat(ish) with not much curve up to themYou need to evaluate without a load in the bed.
That is some good mileage thereThe kyb rear shocks were still the originals on my 2001 tundra. They had nearly 300k miles on them. I don't think the new owner ever replaced them yet. I had to replace the front struts around 150k miles after the struts lower rubber mounting bushings were gone, and it was metal to metal. As far the shocks gm uses...their junk, at 60k miles their toast.
That is one of the many problems with sand bags. They move around.2 bags of 70 pounds each
When I stopped to juice up the taco I checked and both bags had slid forward nearly a foot!
Maybe that is the deal of what was going on back there.
IDK