PryBry,
I've been thinking along the same lines as you have. I don't tow (yet), but my '99 sits perfectly level unloaded. I'm sure that with a load, the rear would sag. Seeing as how you're getting a 3" sag with just a 150 lb. tongue weight, I'm not too optimistic.
I've thought about air bags, air shocks, and coil overs myself, but I haven't yet decided which way to go. Anyway, here are some thoughts about the choices:
Shackles: These will raise the rear a bit, but will not add any stiffness. If you carry a load often, these may not be the way to go. Also, keep in mind that an additional 2" of shackle will only yeild about 1" of lift.
Air shocks: These seem like a good idea, provided you don't go overboard with the pressure. The higher you pump them up, the more load you put on the shock mounts rather than the springs. Still, I think that the auto-leveling suspension used on some Explorers use air shocks. A good advantage is that they are adjustable.
Coil over shocks: These will also add stiffness, but like air shocks, will add load to the shock mounts. They aren't adjustable, but will reduce sag. I don't know how they would affect handling.
Air bags: These appear to be sturdy, and are adjustable. They won't put any additional load on the shock mounts, since they use their own mounts. I've thought about these, but my concern is if I go off road. I'm not sure how much articulation can occur. If a wheel goes too low, I'm afraid that the air bag could be damaged. I honestly don't know enough about them.
Add a leaf: Will add stiffness, as well as load carrying capacity (I wouldn't exceed the manufacturer's rating, though). They aren't adjustable, but they should be durable. I haven't been able to find out if they will fit my 3" (2 doors have 3" single leaf springs) leafs or not.
Helper springs: Should give an effect similar to add a leafs, but can be removed or re-positioned fairly easily (10 minutes). You can put them on when going on vacation and remove them or reposition them for less lift when you are unloaded.
Well, I probably didn't answer your question, and in fact, I probably raised more questions. Since I'm considering firming up the rear suspension as well, I thought I'd share what I am thinking so far.
BTW, if you go with a stiffer spring, you will probably want to change shocks as well. Something like the bilsteins or edelbrocks would probably work better with the harder springs. Remember that shocks really control the oscillations of the suspension more than anything. A stiffer spring will have stronger bounce to it, and will easily overcome the weak factory shocks.
------------------
Tom
