Recommendations on new coil springs for 1992 Trooper???

I have a 1992 Trooper LS with 155k miles. It is in great shape and runs very well. Over the past year, I have noticed a tendency to
really "bottom out" upon hitting a bump. I am a musician and carry around a lot of heavy gear in the back. The frame is apparently hitting the underside of the body when loaded down and I hit bumps. Rather annoying and a very discomforting sound. I replaced the shocks earlier this year and this did not really make a difference.
I'm told that the coil springs may need to be replaced. Does anyone know much about doing this and if there is some standard type of spring to use? I may want to put heavier duty springs due to the load.
Any recommendations are appreciated.
Regards, Eric
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