Why won't defrost on dash blow out on the windshielf

SOURCE: Heater on driver-side blows barely warm air other vents blow cold

the most probabale cause of this would be a disfunctional hear core if you replace your heater your heater will more than likley work the the way it's supposed to

SOURCE: 2001durango vent only blows air through face vent and defroster.

the vent door is broke in the heater control box under dash

SOURCE: ac will not blow thru ac vents, only thru defrost vents.

if vent doors are operated by vacuum check for loose or broken vacuum hoses in the engine compartment , or check the fuse box for any ac/heat fuses if it is a electricaly controoled system

SOURCE: air only blows on floor with very little coming out the defrost or regular vents.

Hope you have the same set up, sure you do. You may have either crossed your vaccum lines or when you were putting your heater/ac box back together or you may of pinched the vents somewhat closed. Cycle through the different zones to ensure the vents are moving as your are selecting (defrost, floor, ect...) If you think the vaccum lines are correct then you may have a reassembly issue with the Heater/ac box. I had the same problem with my 2000 Dodge Ram 4x4. Use the KISS rule "keep it simple stupid" start with the vaccum lines and all your wiring to the motors, make sure everything is plugged in. Hopefully you'll find it there before going back into the box again.

SOURCE: when i have the heater/defroster on the heater

It sounds like you have a blend air door stuck. This is in the housing of the heater a/c blower unit under the passenger side of the dash. There is a motor on the top of the housing that could possibly be stuck or on working.