Ok, my 98 f150 XLT back end it not level. The driver side droops about an 1 1/2" below the passenger side. I figured this happened when I was hauling about 900lbs of cement in the bed and all the weight must have shifted. The only thing I notice is on the leaf springs: the lowest spring (the short one) doesn't have as big a gap at the ends on the driver side versus the passenger side. Other than replacing the springs, is there anything else somebody could suggest??

Also, I have an Air-Lift Air Ride suspension on the back of the truck to help out when I'm pulling my overloaded trailer...

 

Sounds like you may have a broken leaf on the low side.

 

The gas tank is also on the driver side. My 97 regular cab, long bed carries 30 gallons. Carrying that weight for 7+ years has got my drivers side doing the same thing. I'm cheap, so I've been thinking about just swapping sides with the springs.