One other thing to consider is tire pressures. They can make all the difference in the world, too.
One mistake I see often is folks expecting to load up a rig and run exactly the same tire pressures they have in it when not towing. The simple truth is that having more weight on the back end (or the front end, for that matter) than the tire pressure can handle will cause squirrelly behavior.
I have seen vehicles that used the entire roadway at 45 because they squirmed all over the place due to low tire pressures, turn into perfectly civilized rides at 60 when enough air was put in the tires.
Look on the tire sidewalls, you will find a rating of X number of pounds at Y psi. If you take the weight you expect that tire to carry, divide by the max number of pounds the tire can carry, and multiply by the max tire pressure, you will learn how much air you need in there. This is a starting point. You may need to go to more air than that (up to the max on the sidewall) to get it to behave. But it is absolutely certain that if you have less than that, BAD THINGS will happen.
