[Longhorn Man]

the gear should be a 3.54 in a dana rear axle if it is all original.
150K on the engine is quite a lot if it's never been rebuilt. The more I hear/read about these older big blocks, the less I am impressed with them.
I've personally never towed anything in a coil spring truck and thought it was doing a good job... more like what you describe. new bushings in the trailing arms and panhard bar would probably do wonders... poly instead of rubber since you are asking a lot out of it. (less 'squirm')
Take a good look at your rear tires too, and I recomend inflating them to max pressure.