Tire Sidewall problems of the 2003 Chevrolet Silverado 2500

I just noticed the sidewalls of firestone steeltex tires ( size lt245/75r16 at load range e) cracking in a uniform circle about 2 inches in from the edge of the tread. It looks like they are cracking along a seam in the sidewall of some sort. I reviewed the warranty and it states that dry rot cracks are covered for 4 years from the date of purchase or if no proof of purchase exists, 4 years from the manufactured date the tires came as original equipment on a brand new 2003 Silverado 2500 pick up truck that I purchased on April 5, 2003. I am assuming that date qualifies as the date of my purchase of the tires. I had the local firestone dealer in haysville, north carolina look at them and his position is that since the tires were manufactured between 20000 and 2002, the warranty no longer applies. These people are selling tires that are unsafe and prone to sidewall failure and then will not do anything to correct the problem.