From what i've seen GM trucks have an offset on the rearend about that much....atleast all the GMs i've seen do, mine included. This is also on a varying amount of drops and stock trucks.

 

I went outside a second ago and stared at it and stared at it and stared at it and I think it is just my head.

 

id still try to measure it just to make sure, its gonna bother you until you do

 

I talked to a gm engineer just the other day abought this same thing . this is what he told me almost all rear wheel drive cars and trucks drivelines are shifted to the right to compensate for such things as steering shaft and steering boxs and there was some other thing that came into factor but i just cant remember.(lol) thats what happens when you get old

 

put it on an alignment machine and have the thrust angle checked. this is the relation between the wheels and the centreline of the vehicle.
this is not normally adjustable in the rear but if the truck is lowered and spring clamps have been off or loosened something may have moved.
the trick will be finding an alignment guy/shop willing to fix it,
if something has moved, you could probably fix it on the alignment rack with a willing shop.
is the steering wheel centered? does the vehicle drive straight without steering correction.
been doing wheel alignments for 20 years and have seen lots of weird stuff
good luck