So which parts are replaced? the hood, front fender, bumper, grill.
The items you see on the outside is the “easy” stuff. I say “easy” because it’s got to mount up, line up, have good body gaps, and look like it belongs. You need to think about all the items underneath that you don’t see that may or may not be the same and/or compatible that will have to be changed in order to mount all the body parts you do see.
Bumper cover mounting brackets
Bumper impact bar & mounting brackets
Bumper support
Fender mounts
Wheel well liners
Radiator Core Support, Brace, Shroud, and Hardware
How much of this stuff is the same between the SUV and the truck platform?
How much of this can be re-used or will require to be replaced in order to mount the equivalent truck body parts to the SUV?
And since you didn’t mention them; Headlights. Sounds simple right; Maybe maybe not?
GMT900 Yukon headlights use a dual filament bulb for both low and high beam. Those Sierra headlights use 2 separate bulbs; 1 for low beam and 1 for high beam. That will require a new headlight wiring harness. Could you just swap in the sierra headlight wiring harness? Would you have to make your own custom headlight wiring harness? Would it also require you to re-pin a wiring harness connector? Would that also require modifying the BCM or other vehicle module so all that communicates and works?
These are the types of questions you’ll have to research confirm and have the answers to before tearing your truck apart. Maybe a lot of the answer are yes they’re the same or compatible but you’re not going to know that until you take a laundry list of OEM Yukon SUV front end part numbers and compare them to equivalent model Sierra Truck OEM front end part numbers.
@kbuskill is right the lines for the front fenders don’t match the rear exactly from truck to SUV. The fenders flares on the SUV pic posted help hide that fact. In fact the front to rear fender flares on the SUV posted don’t even match if you look closely. The angle of the flare out on the front fenders is much more sharp and boxy, the rear fender flares angle is softer more rounded and doesn’t protrude out as much. The fronts are for a truck and the rear are for an SUV due to rear wheel well opening and having to account for the rear doors being a part of the wheel well opening.