Towing help - again

First off, Thanks to all for their comments. Weather I like the outcome or not, it is what it is. The vehicle is a Gas engine. So my concerns or questioning as well as others. Why could the same vehicle with different gears or a diesel engine haul that much more weight. anywhere from 2 to 4 thousand pounds. I am still a bit confused. My previous vehicle had a towing capacity of 6400 lbs. and we towed around about 7200 pounds before we really got into the weight thingy. But we did it, yes up big long hills at about 25 to 40 MPH and after 160 K miles on it, had proceeded to go through a rear end, an engine and a transmission. This all at around 100 K miles. So I am almost inclined to by the trailer we like, shipping weight of 9255 lbs. and a carrying capacity of 2915 lbs. deal with it for a bit and then either trade in the truck or upgrade the gearing to 4:10. or what about 3:93 gears. anyhow, my info and I appreciate the input you all have provided. Additional input is welcome and appreciated. Good Luck & Be Safe ! ! !