Ton ratings
The ton rating is not a measure of how much weight your truck can carry, exactly. There's been a fair amount of debate as to where this designation really came from, from old wagon ratings to the military.
I've read that the military set the rating to be a very consrevative underestimate of the actual capacity of the truck, typically 50%, to ensure safe travel on extremely rough roads. On good roads, twice the ton rating, or the actual capacity of the truck is permitted. This is still significantly under what it'll take to break the truck.
I've often carried 55 85-90 lb hay bales on my 1-ton, which has a big heavy steel flatbed on it (truck weighs 8380 empty). That's pushing 2. 5 tons on the truck bed. It's down on the overload springs, but there's still a couple inches of travel left in the suspension. It won't take corners fast (due to the high center of gravity) and I don't presonally like going over 45MPH with that kind of load, but it does it just fine.
With a 3/4 ton, I'd probably feel safe running at 2 to 2. 5 times the ton rating, or 3000 to 3750 lbs. Just remember to give yourself extra distance to stop the extra load, you WILL notice it takes longer to get stopped.
-cj