2008 GMC Sierra 1500 4x4. When in 4x4 the right front wheel does not spin.

Horn is under the front headlight light, which would have to be removed to access or pull the front wheel well liners out and look in just below the running lights. I believe 2 horns.
2008 GMC Sierra Horn Location

horn location

Chevy Silverado and GMC Sierra Fuel Pump/Fuel System - Edmunds.com

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SilveradoSierra.com • Cranks but wont start. Fuel problem ...

4X4 actually provides power to the front 2, and the back 2.
Unless the back has limited slip or posi-track, if 1 wheel spins, the other will not turn.
Since the front wheels have to turn at different rates to accomodate turning radius differences, there is (usually) to limited slip on the front differential, so if one wheel spins, the other will not turn.
So, if you have one front wheel spinning at the same time as one back wheel is spinning, and you don't have posi-trac, you're stuck.
Cheerfuls.

If 4X4 Check Outer CV Joint On Right-- Or Housing Bearing-- If 4X2 Check R/H Wheel Bearings

Balance tires,check rotors and tie rods.

Sounds like you need a front axle actuator. This actuator bolts to the side of the front differential housing. It is electrically engaged. You can purchase them online starting at about $40

have your front end components checked, and have a front end alignment performed, double check your tire pressure.

Yes it will spin.If the front tires are spinning so is the shaft.

if they only stoped working on the passengers side I would check the wirring going into passengers side door for a broken wire, or right at the passengers side kick panel.

transfer case