Welllllll. If it's a Craftsman do they still have that awesome lifetime guarantee? Then bring them back. 3-4 years is not that old but it depends on the number of charging cycles they've had and if they are NiCads it could be a memory issue.
If you otherwise like the drill go to
www.BatteryBarn.com and look up replacement batts. I've been very happy with the after market panasonic replacements. The only thing is you need to be sure that you order the correct kind of batts for your charger. You can't charge NIMh in a NiCad charger and vice versa. But your drill won't care. Get the greatest Milliamp hour rating they have.