I have a 1991 Dodge diesel 3/4 ton (single wheel)automatic transmission and tow a bumper pull horse trailer with 4 mules. Without having weighed the trailer loaded, I am guesstamating the weight at approx 9,000 lb. I don't know the GCWR of my truck (bought it used) and wondered if anybody knows of a website where I can find info on older model Dodges. Also, I'm thinking of buying another Dodge (1992 a 1-ton dually, automatic) but this has a GCWR of 12,000. Apparently this truck has 4:10 rear axles but I haven't heard of an automatic with 4:10's. Seems like I may not be gaining much in the GCWR. I love my Dodge trucks especially the older models but wondered if I'm going to have enough truck for the load I haul? I don't carry a camper with the mules but do load up the back of the truck with gear.

Hello BC .. Welcome to the forum...
The only difference a dually gets a person is the carrying capacity. The same engine and transmission in a 3/4 ton truck has the same CGVWR. So should every one get SRW trucks for their monster LQ horse trailers? Hello... NO!
Those big GN trailers put a lot of weight on the hitch. Air bags, helper springs and the like don't change the ratings of the tires, wheels, axles, shocks, etc.
The dually wheels also provide a lot more sway stability. The eight tire sidewalls flex a lot less. The springs aren't bottomed out either.
You might closely estimate how much gear you're going to haul in the truck and decide if the dually is for you.