According to the 1500 specs page which I downloaded from Bigfoot before it was discontinued, it is a 3500 lb axle (in both 15B17CB and 15B17G variants). I can't be sure of the model year... I believe that it was the last version posted before the 1500 series was discontinued, and I downloaded it in October 2004.
and for a bonus, the other 1500 stuff which I grabbed at the same time (while I'm at it)...
Yes, I believe that the 2500-series model (25B17.5xx) has a single 4400 lb axle, while the large units have tandem 3500 lb axles.