more trction in the rear of my scania

[cosworth34677]

OK I've got this scania r470 2x4 semi and kind of love the little guy, its my go to truck, but the one thing that make my sick is the lack of traction in the rear of the truck. even on concrete and assault is will spin one of the wheels, also its pretty much useless on dirt unless there is a heavy load in the back. is there a way to make it have better traction? i don't want to lock the rear axle because i drive it on sidewalks and the road a lot and don't want to mess up the diff.

i thought about putting some fishing weights on the back and maybe more metal parts on it, but before i put tons of money in it what do you guys think?