Chances are its already over the rear tire ratings. Older rigs had shorter wheelbases which put more weight on the rear axle. That problem can be mitigated, but only somewhat because addressing the tires doesn't put more weight on the front axle, which is the source of many handling complaints. As J-D mentioned, you really need to get it weighed to find out the full story.
Also, older rigs tend to weigh more than the newer rigs with more slights because manufacturing methods and materials have made some big improvements over the last 10 years.