I have a 98 f150 4.6 2 wd my ac blows cold out of the defrost only!!!! And when I hit a certin rpm it goes to hot heater did the same but all the vents were workin at that time also

 

Search for EATC repair, there are a few threads on here about it. Or it could be a vaccumm leak.

 

When you start the truck and let it run for about 30 sec do you have air at the dash vents? If you have your fan on low speed and switch your mode dial from defrost to vent to floor do you hear a change Like a vent door moving? If yes to all of the above. Then I wil ask you as you are driving do you not have any dash vent air normally or just when you are excellerating? If you normally have air at vents but loose it on excelleration check that you vacume check valve if functioning. should be tucked back near the fire wall on the passenger side. If you never have air at the vents check the vacume lines going to the check valve while engine is running to see if you have vacume at the line. carefull!!!! of the fan and other moving parts. If you have good vacume there check inside under the glove box where the vacume line comes in through the fire wall do you have vacume there? from here it gets a little harder. (you may also have a busted vacume line going to your vacume resivour)
hope this helps.