Struts? Or coil springs?
#2
It is the shock absorbers that take some of the up-and-down motion and reduce the amplitude to smooth out the ride.
What you probably need is new shocks. Have you visually inspected them? Sometimes it is pretty obvious they are gone by oil leaking out of them.
#6
I had my one of my front shocks go out by 130k miles and 18 y.o. so it is pretty reasonable to expect them go out by 150k.
#10
FWIW, my Blazer had 291,000 miles on it when I got it, and it still had the original shocks. I don't even know how the previous owner managed to drive it. It was seriously like piloting a boat on rough seas.