I added new Leaf springs and now the suspension is leaning

I'm not much of a mechanic but I'm trying to beef up my truck a little bit. I recently put on helper leaf springs to the rear of my 2001 nissan frontier and they helped tremendously with the sag and soft suspension. As time has gone on, however, I have noticed a continuously greater tilt on the entire left side of the truck. When I measured the gap from the top of the tire (both front and rear) there is a 4 inch difference between the right and left sides. I thought I might have been the helper springs, but the tilt is in the front as well and the rear so I don't know what it could be. Any ideas or advise would be greatly appreciated. Thanks for your time
Tucker in California