The rear shocks have a preload adjuster near the bottom of the shock. On my 05 it was just a hole in the chrome cover, but yours may be different. You need to add preload meaning shorten the spring. It probably goes up by steps with a maximum of maybe 4 or 5 steps. Use a C spanner and rotate clockwise and the collar should step up on a ramp. Add three steps to both sides and take for a test ride. If still bottoming out 2 up, add another step both sides. If it's maxed out and you're still bottoming the suspension you need to replace with stiffer shock springs (unlikely). Upgrading the shocks with aftermarket high end units will provide more adjustability since you can also dial compression and rebound damping that's unavailable on the stock units.