When I bought my EXT a few years back, I was getting a Service Suspension error in the DIC.
Turned out both air shocks were blown out and the air compressor was toast.
So the truck presented a "somewhat" level orientation front to rear since there was no lift from the rear air shocks.
I measured the rear height before and after the new gear was installed and found the rear was raised close to 1.5 inches.
I suspect you are seeing the same now that your suspension can function as designed.
What you are seeing is most likely the was it was when it rolled off of the assembly line.
There is nothing that says you can not apply some of the "leveling" techniques described elsewhere on this site, if you desire a different look.
All of these trucks are designed to sit rear high.
Not everyone likes that and they mod their truck to achieve the look they prefer.
Doesn't bother me, so I will just let mine bump the way it is.