Weight has a huge effect on performance. Not just on the street, but offroad. In some cases weight is good, in others its bad. Unsprung weight is almost always good offroad, but not for the street though.

One major concern when adding lots of weight is axle strength. The stock axles are rated for X amount of weight. As you approach that amount, your axles basically become weaker. So there is a limit where you need to take a step back and think about how well the drive train is going to hold up to all that weight. This is one of the reasons most people with heavy offroad vehicles and large tires move up to 1 ton axles. Full float axles handle the weight fine, and easily handle large tires.

My old '75 Cherokee weighed in at 5200 lbs, and although I never did break any axles, I went through a whole lot of u-joints.

 

A slight departure, but I've been reading about how people spray bed liner to protect from rust and such. Anyone know how much that stuff weighs? Seems like it's would add significant amounts of weight to the whole body? I don't know I'm not really familiar with it.