99 tahoe, rear air is cold, front air is warm, has

Did you then draw a vacuum and charge the system?

I would agree it might be the valve.
One line should be cold and the other warm or hot.

If it is cold in the rear, that eliminates the "freon" part of the system. (It apparently has enough freon and the compressor is apparently working.)

So that leaves you with a blend-air door problem on the front A/C unit. That may be a blend-air door actuator, a broken blend-air door, or a faulty control head. (Proper diagnosis required)

I just worked through a similar issue with my van. The front and rear systems each have their own expansion valve. Chances are the front one is stuck and needs to be replaced. System will have to be evacuated and recharged.

I just took my 04 Tahoe in for the same thing and it was the a/c actuator.

That "thing" is part of the compressor clutch. When it’s spinning the clutch is engaged, when it stops the compressor is off. If it’s constantly cycling, you probably don’t have enough freon in the system. But remember, however the gas escaped before, it will escape again, unless you fix the leak.

check the freon level

this is caused by a defective temperature control blend door actuator motor. Don't overhcahrge the system, if one part of dual climate works ok the problem is not the compressor or the system charge

you may just be low on freon. The rear will often work ok when low, but the front won't

The same compressor serves both front and rear systems. Check that first.

ALso, check to see if your front blend door rotates when you raise & lower the temperature.

report back on results

The problem might be that you got out of freon gas.