Sometimes when you replace an alternator, the battery is down; when you start up with the new alternator installed, it has to put out a very high current load for the first few minutes to recharge the battery. This can stress the alternator and lead to early failure later. The best technique is the charge the battery *before* the first start with the new alternator.
Some of the aftermarket shops have lifetime guarantees on alternators. Replacing one isn't very hard.
How many miles in the two years?
Have the charging system professionally checked--Sears will do it for about $10.
Also: a dying battery will cause your car to go through alternators more quickly, as they do not hold a charge, and thereby are constantly demanding charging current from the alternator. If the battery is weak, it should show up in the charging system test. If there is any doubt, change the battery.
I have gone through this several times; the only two batteries I have found that last well are the factory GM battery, and Sears DieHard. The top of the line DieHard is 1/3 the price of the GM battery... you decide. Their charging system check is also a fraction of the dealer price.
Another possibility: corroded terminals at the battery. They make for poor connections, thus lower charging voltage, and sort of simulate the conditions created by a weak battery. I pull into sears at least once per year, and have the charging system checked and the battery serviced and the terminals cleaned. Seems to help...