This may seem like a minor issue but i was standing behind my truck today and noticed the back of the truck has a slight slant to it. I measured the difference and it's about 1 1/2 inches lower on the left than the right. The bed measures the same height behind the cab, so the difference lies somewhere between the back wheels and the tailgate. Does anyone know the cause of this or better yet a fix? I know it can't be perfect but this is really bugging the crap out of me now that I've noticed it

 

I have noticed that on my truck also. Never really put alot of thought into it, but it is annoying that it isnt level.

I have a regular cab short bed 2X4

 

I have noticed that the left side of my truck is lower at times too.... When I have a FULL tank of gas. The gas tank, located left of center, really makes the left rear squat down. I think that there maybe a difference in cab height where you measured, but is was so small that you didn't notice it. By the time you got to the bumper the side of the truck acts like a huge lever, and shows the actual difference in height of the truck.
This MAYBE the cause for your leanin' machine... Measure at an empty tank once and see what the difference is.!
MK

 

I bought a 1977 chevrolet, brand new, that had the gas tank on the right side. I drove it for 12 years and it always leaned one way, depending on how much gas that it had in the tank. Haven't noticed my 04 F150 leaning. I will put the machinists level on it tommorrow.

 

good call mkoser. I dont know why I didnt think of that. Seems like the first thing anyone should think of.

I'll check into that

 

Remember sometimes the obvious isn't so obvious at all. I am not certain that is your problem 100%, but it is worth a look see...
MK