Blend door

I am assuming its' the rh blend door, you need to pull the audio control module out as well as glove box to reach it. There is also one on lh side so I I assumed rh. You can check with a independent garage for a cheaper price

Does your truck have the automatic dual zone heat/ac system? If so and the issue is that the drivers side temperature doesn't change when you switch from hot to cold the LH blend door is the problem. That requires that the heater case itself be removed to access it. That basically means then need to remove the entire dash panel to get to the blend door controller as it is mounted to the rear of the heater box against the firewall. Book time for that repair is around 6 hours.

If you don't have the dual zone controls and have just the manual heat controls then you only have the blend door actuator that HMAC told you about, the book time for that is about 2 hours.

In either case you could very likely do better at an independent shop on pricing, but at dealer labor prices 800 would probably be correct for the LH blend door.