Here is some more info for the curious: 

The tank is a Surge, which was made by Mueller (they have been building the Surge tanks since the early 90's).  The tank has two 5 hp compressors, with each running one side of the tank. 

We originally had a 1500 gallon Surge tank that we installed in 1988.  It worked great, but we needed a bigger tank in 1995.  We installed a 2000 gallon Surge, but it took twice as long to cool as our old one.  This seemed weird, as the bigger tank had more plate area, and we figured it would cool faster.  We ran tests every day for weeks with different amounts of freon, trying to get it to cool faster.  Each time we calculated the BTU per hour and could never get it above 75% of the rated BTU of the compressers (the first time it was at 50%).  The dealer decided to order another new tank.  We installed it and it was worse!!!!!  The best we could get was 65%.  They did some testing at the factory and found "insufficient flow" on the Surge tanks (the Muellers were ok).  They determined that they were not getting the plates expanded enough.  We had the option of "expanding" the plates on our current tank or getting another new one.  We opted to get a new one.  So the third tank arrived a couple months later (1996) and was running at over 90% efficiency right out of the box.  That is our current (leaky) tank.

Last week we evacuated the freon and pumped up the tank with 200 psi of nitrogen.  In about 10 minutes it leaked down to 175 psi.  Checked all external connections, but did not find a leak.  We pumped the pressure up to 225 psi and I went inside the tank with a stethoscope.  I could hear the leak near the front of the tank.  I then used a "listening" device with a series of LED lights to find the exact spot.  The "listening" device has a little tube that you move near the surface.  As you get closer to the leak, the lights start lighting up as it picks up more sound.  This also works for finding leaks at external connections.

As you can imagine, I am very leary of buying a new tank based on our experiences back in 95/96.  Especially if it is the same design (And our dealer says that it would be the exact same tank).  Repairing the tank seems to be the best option.  I just need to locate someone with experience to do the repair.  Thanks for all your suggestions!