heavy load Rear Suspension?

[cosworth34677]

i been wondering if there is a way to get the rear of tamiya trucks to carry more load? i have now two dump trucks and bottom out way before the truck is loaded. i not wanting to over load the trucks but i do live in florida and heavy wet sand is kind of a normal thing, our water table is really close to the surface.

i put beach sand and rocks in the trucks, the drive train seen to be fine the the suspension is like " oh no" so any ideas? i still want some kind of movement i do drive on the road and at a good speed, well for a truck. any would be great!