I agree that your load is a bit much for the Tahoe. Everything will be strained to the max and is unsafe. However, that not what you asked for.
You didn't mention what you meant by "horrible". Bouncy? Mushy?
ANY new parts on an old suspension is going to give a firmer ride. You can't have a Caddy ride and still safely tow/haul high loads. A good compromise is on-demand suspension components like air bags to boost capacity when you need it and allow you to relax things a bit when just tooling around.
Body roll can be improved with heavy anti-sway bars in the front and back. Use Poly bushings to make the system even stiffer. The ride will suffer a bit because a bump on one side of the truck will be transmitted to the other through the bars. The wheels will not absorb the bump itself as they will be held tighter to the frame.
Heavy shocks and springs also affect roll as the suspension will be stiffer overall. New springs to replace old saggy ones is fine. A longer spring of the same coil diameter (new springs compared to old) will give you more height with about the same ride. A HEAVIER spring will give more height AND a harsher ride and the suspension will compress less loaded and unloaded. A rigid coil spacer of the kind that goes above or below the spring changes the truck height only. A rubber coil spacer will act as a mini spring by compressing a bit with bumps but can also contributed to the dribble effect as it will compress at a different rate than the spring. This bit is hard to explain.. Say we hit a bump and the coil spring smoothly compresses and relaxes 2 inches. The rubber spacer, being smaller and stiffer, added to the mix will compress and rebound many times (and many times faster) as the spring is still compressing setting up a shock wave that vibrates through the spring like a tuning fork. Since these vibes are so short, the shocks won't damp them out. Just something to be aware of.
Shocks are just motion dampers. A lighter shock will ride a bit smoother because it allows the spring to flex and the wheel to move to absorb bumps. If the shock is TOO light, the wheels will dribble like a basketball and the tire will spend more time in the air than on the ground leading to bad tire wear and loss of traction. If they are too heavy, they won't allow the springs to do their job of smoothing the ride.
All this just leads to the point that setting up a good suspension is complex and more than bolting on the latest shock fad.
Set up your unloaded suspension as firm (I am assuming you have mushiness) as you want to go for daily driving and add adjustable air bags to stiffen things up only when you need it. Use a weight distributing hitch package and make sure the tongue weight is set properly.
The Tahoe has such a short wheelbase that even the spacing of the concrete slabs on a freeway can set up a harmonic while towing that will make the truck do the bunny hop going down the road. Had I considered that when truck shopping I would have gotten something longer to pull my boat. Bunny hopping is not fun.
I am sorry I can't provide more help without experiencing your setup but I hope you can use some of this to get a better result.
Ed
