Rear Suspension problem.

I have a 2008 Prius with 102k miles and the rear end is really low.
I do use it for work but I don't carry particularly heavy loads.
Problem is the back end is so low, when I go over the pad type of speed bumps, the exhaust scrapes.
Here in the UK these cars are used as taxis, and carrying 4 people is much heavier than anything i carry?
Can anyone offer a solution? I know you can get spring assistors.. they are like pieces of rubber that sit between the coils.
Any advice welcome, thank you