Bruce, go and measure your fender height front and rear (from fender lip to wheel centerline, NOT to the ground), the driver side should be slight lower due to weight of driver and fuel tank on the driver side, the passenger side should be about 1/2-3/4ish higher given the age of your truck if under 1 inch you should be ok with just replacing shocks, and Bilstein 4600 or 5,000+ series are great shocks, much higher quality than OEM, and Bilstein's are my recommendation.
little food for thought, shocks are rated at 80,000 miles the fact that you have 150K on stockers goodness. 80,000miles is recommend time for replacement, but not necessary as you already know.
I have a 2013 AWD Sierra Denali as well, you should visit the truck forums, there are folks with much greater details & experience about GM trucks than this site by a long shot.
the rear shocks are super easy about 20 minutes for replacement, the fronts are combined shocks and springs, and come out as 1 unit, then the shock has to be removed from the spring.
Since you have Sierra Denali 1500, its equipped Z-60 suspension system ( Suspension Package, High-Performance {truck sits lower, larger front sway bar, mono tube shocks, and low profile tires to match the 20" wheels, and better suited for street performance & handling over the standard suspension}) if springs are needed I would stay with OEM springs, but Bilstein's shocks
also ping me if you want to know more about the GMT-800/900 chassis', you can add 3 extra security sensors to your truck, shock, tilt and tailgate to reduce theft.
I made a few KB's on the site I visit the most. (I am camcamaro1991 on that site)
Dust filter retro fit kit, and will work on all the GMT-900 trucks/SUV's
http://www.performancetrucks.net/fo...bin-air-filter-retro-kit-gm-oem-parts-533295/
GM illuminated door sills for GMT-900 trucks/SUV's
http://www.performancetrucks.net/fo...-oem-521660/?highlight=illuminated+door+sills