Upper vents blowing/leaking air all the time.
 

Can't turn off, can't switch to lower or defrost vents. I'm guessing it's a vacuum problem? Where to start?

Sounds like a vacuum problem to me! Just start replacing all of the vacuum lines on the truck its not a bad thing to do and its pretty cheap. Keep in mind that there will always be some air coming through the vents no matter what. I don't know why thats just the way these trucks are.

Start under the hood, look for dry rot and disconnected lines.

ok inside the the dash where the heater controls are is alot of tiny vacum lines. now these lines are soft and dont rot out but in the switch there is a seal and it might leak. start the truck. put your ear up to the switch. switch it to defrost and back and forth through all the settings. do you hear hissing when it changes? if you do then its getting vacuum to the switch and one of the hoses must have popped off elswhere in the dash which is unlikley. if you dont hear a hiss at the switch then pop the hood and on the driver side you should see a black ball hanging. run the line down to where it enters the firewall. now there should be a small black hard plastic line there somewhere. check to see if its disconnected or just broke. usually them hard lines will break when entering the firewall.

oh and if your hearing a hissing all the time at the switch in all settings then the hoses came loose behind the switch or like i said it could be the seal inside the switch. most likely they just popped off behind it.

I do hear a hiss but only with the AC, not the heat.

ok if you hear a hiss at the heater controls i would take the controls out a little ways and make sure all the tiny vac lines are connected. make sure there fully seated. if they are it might be the swtich itself is broke. i would go to a parts store and rent a vacum guage. you will get all your money back. and hook it up at the tee where that black ball is at. might have to make an adapter if the kit dont come with one. start the engine. should be around 14-19 psi. and hold steady not jumping all over the place. then turn the truck off. it should hold that pressure. if not then you have a vacum leak.

oh when you have the gauge hooked up you might have to buy another tee and some line to hook up the gauge. leave nothing unplugged when your testing. then whith the truck off and pressure holding steady at the gauge move the heater control to a different setting. the pressure will go down a psi or two but does it loose all pressure? might have two pump up the pressure with the gauge to try all the settings on the controls. if a particular setting is letting all the pressure drain out it might be the vacum line running the vents popped off in the dash or the switch could be bad. and dont pump it up more than 20psi dont want to damage anything.

Leaking
 

Definitely a leak, holds at 19 running and drops to zero as soon as the key is turned off.
Looks like I'm buying a roll of vaccum line.
PS-Mabey not on the driver's side firewall there's a line that goes through the firewall.
If I put my gauge on that T in place of that line it holds vaccum.
Pretty sure that's a hvac supply line, going to tear apart the inside this weekend and see what I find.