I get the same thing with my '94, it's sprung pretty heavy and too light in the rear. It can almost change lanes when you hit a roll in the asphalt or a good bump. I've gone all over my front end and suspension, but it's really done it all it's life. It has only a front sway bar and a rear bar would help the cornering, but I doubt it would do much for the rear end bounce. (I had 1200lbs in it the other day and a 1100lb trailer behind it and it only sat about an inch lower in the rear), that also says something about the way it's sprung in the rear. It handles weight better than some full sized trucks I've had.
I didn't by this one new, but I'm assuming that its all factory original. It does ride a bit harsh, but I wouldn't want to sacrifice and payload capacity for a better ride. My GVW is labeled at 4700, and the payload rating is 1650.