I bought a '99 Audi A4 Quattro wagon a year or so ago, and I can see that it needs two new tires. The tire shop near my house says I need to buy all four new, though, because it’s an all-wheel-drive Audi. Is that right?
AWD vehicles typically need to have all four tires pretty closely matched in diameter. Your owners’ manual should address this. Follow the guidance of the manual.
That could easily be the case, most AWD vehicles tires that are matched as closely as possible.
not sure how you only need 2, are 2 very very close to new?
Yes, AWD vehicles need to have 4 tires that have the same diameter, otherwise damage may result to the drive train of the car.
Also, you should rotate your tires more often on an AWD vehicle to keep the wear even on the 4 tires.
BC.
First, see if your manual requires the tire circumferences to match. If so, then either you should buy four tires or you should buy two tires from a place that can shave them down to match (but you should do the math to see which option works out more cheaply). However, if they are required to match, you may have already done some damage with the current set of tires.
Yes the tire shop is right, because your Audi has AWD.
If you only need 2 tires is that because two have little tread left and the other 2 are in good shape treadwise? Or, did 2 tires get ruined because the car was out of alignment? If you are currently running non matching tires on the car you may have already caused some damage to the AWD system.
Buy 4 tires, that is much cheaper than repairing the AWD system which is full of expensive components. Keep the 2 good tires as spares or sell them to a tire shop that handles used tires.
I disagree with everyone. You do not need to buy 4 tires; just shave the new ones to match the remaining old tires. Or just buy 4 new tires. The older ones won’t last that long anyway. You might consider rotating tires with the new set to even out the wear.
I’ll go with the ‘shaving the new tires’ idea. Might as well get all you can out of the old ones… 
I couldn’t resist…
“You might consider rotating tires with the new set to even out the wear.”
The OP might also want to consider reading the Owner’s Manual, which definitely addresses this issue.
If the tires are marked at the 6:00 position with the wheels straight ahead and the car driven until the wheels have rotated 10 times you can compare the difference in the rotation from wheel to wheel.
This article has the info you seek >>> http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18
Scroll down to the bottom for Audi information.
The problem here is how do you express all the possibilities in a single statement, simple enough for tire shops (who are not rocket scientists) to remember.
Clearly, AWD systems differ. Some are very sensitive to diameter, some are not.
To further complicate the matter, tires are not always built to the same diameter as indicated by the “size”. Even Tire Rack doesn’t get this right.
The safe way to do this is to change all 4 tires every time an AWD is encountered.