issue but no codes.

[hops]

2005 2500hd 6.0 4l80e 4x4 - stock except 285/70/17 tire and wheels.

52k, purchased new.

No cel.
No codes.

Perceived lack of power. Still goes but seems flat.
Terrible MPG. (6.8 in town.)
Sounds "congested" to me for a lack of better term, but OK from the back.

What would cause something like this and not throw a code?

Of coarse I can't find a competent shop.
I must have sucker written on my forehead.

Give me some ideas!?!