You make it sound like adding rear seats has made the back of the car heavy - the car had back seats when it left the factory, so unless you've done something really strange, the seats are not related to the problem.
We can't see the vehicle, so you have to tell us what is going on, you have to tell us what has been done, how was it lifted - four inch longer springs, two inch body lift, two inch spacers - where did they come from, we don't know.
At this point, I'm going to guess that your rear springs are too soft and that's why you have a problem with it bottoming out - but - that's just my guess.