'08 XLT here with the AC blowing through the defrost vents. Thanks for the thread, really helped narrow it down. The dowel rod quick fix worked and is fine for now, but I'll want to fix the root problem myself if I can.
On my vehicle, AC seems to blow equally distributed between defrost and dash vents on startup at AC level 1. However as I increase the AC to 2nd or 3rd setting, it moved to the windshield vents and stays there. I popped the dowel in the open the dash vents and it works.
Two different mechanics recommended the blend door actuator which I bought for $20, but didn't install. As I understand it, this has nothing to do with the vent doors, it's about temp. And temps are ok. So skipping that for now. I also bought a vacuum check valve and will replace that simply because it's cheap and easy. I'll see if that does it.
If not, next will be pulling down the glove box and checking the arm to see if it snapped off as recommended by
@TexasArmadillo. If so it's the zip tie method.
After that I get into the vacuum actuator behind the center console that
@TechMike talked about. That one I don't think I want to try to do, so if I end up there it's probably permanent dowel rod. But hey, the AC blows out the front vents as summer is coming up, and that's what I needed.
Thanks all. Good stuff.