Teleconverter and AF troubles.

I have a EOS Rebel XT and recently bought a tamaron 2.0X teleconverter. I needing the extra zoom, and didnt want to spend another $1,000 on a lens. I attached it to my Canon 70-200mm 2.8 L and the AF is all wacked out. It's like the focus can't lock on. I'd manual focus but I really need the quicker option here. Is there anything I can do?

Cheers,
-E

Could you be a little more specific about how it's behaving than "all wacked out"? Does it hunt? Zoom to the wrong focus point? Focus very slowly?

There is a minimum light level for AF to work. It uses contrast to locate the focus and if its too dark it can't find the right spot. The minimum level varies from camera to camera with the 1D series being best, especially the centre AF point. I am not sure what it is a for an XT but the number doesnt matter much anyway, you only need to know that there is a limit.
Your F2.8 lens has a wide aperture (2.8 ) when its focussing so it should get plenty of light into the camera. This means that it will autofocus quite well even on a dull day or in the evening. But if you try it in a dark enough situation AF won't work anymore.
When you add a teleconverter it makes your lens max aperture 2 stops smaller (5.6) This means that it gets only 1/4 of the light to the AF sensor so it needs to be 4 times brighter for AF to work. Usually AF is still OK on sunny or even average days but I wouldn't be too surprised if it doesnt work in the evenings or if its a dull day etc.

If it still won't work in full sunlight maybe you have another problem.

I used a 2x TC on my 70-200 f/2.8 and didn't have that much of a problem with the AF not working. Sometimes it would hunt a fair bit but more often than not, it worked ok. Even on AI Servo mode.