Rodeo hitting rear stops

Thought I asked this before but maybe it didn't get posted...
My truck is a 94 Rodeo. It has Rancho 5000 shocks on it. But my problem isn't rebound, the problem is that the rear end bottoms (hits the rubber stops) when going over speed bumps.
The part of the country I live in has taken to putting speed bumps in neighborhoods and the college campus also has them. They are designed to slow people to 20 mph. But even slowing to 20, they cause my Rodeo to smack the stops.
My thought is that if I had springs rated for more weight I wouldn't have the problem.
Will helper springs help? If I use helper springs will the ride be much harsher?
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