sagnok

Hi, I am a newbie. I have a '99 model LX470 with 245,000 kms on the clock. I am looking to purchase a new caravan but have concerns about the towing capacity of my vehicle. Just by a per chance event, a friend recently asked me to tow their new 24' Jurgens (lightweight) caravan which has a tare weight of 1600 kgs (unloaded) and I discovered that my Lexus began rocking (not swaying) over some uneven road surfaces. This experience now cautions me when choosing to buying a new van as the vans I have been looking at most often have tare weights of around 2.5+tonne. Has anyone had any similar experience and can suggest what I can do to firm up the suspension so that I can tow a more heavy weight caravan? Cheers.