I have an 87 D-150 and my friend has a 90. Both trucks lean slightly to the drivers side. At first it caused a great amount of concern til I realized they both have the large fuel tanks and the lean is more noticeable when the tank is full or near full.

I suggest you do what I did. I named my truck "EILEEN" problem solved.

 

that makes sense, and it fits with my mechanics theory that it isnt one thing bent, but everything just bent a little adding to around an inch or so.

however; if this lean occurs with 4x4 versions then it isnt the issue since they have a rear mounted tank.

ive given up anyhow.