This info was found in the 2006 Chevrolet Truck Order Guide.
The 1500HD was the old 2500 (non HD) replacment. At one time GM offered a 8600 lb 3/4 ton and a 9000 lb 1 ton single rear wheel truck. They came out with the 2500HD at 9200 lb and a 2500 at 8600 lb. The changed the 2500 to a 1500HD.
The 1500HD was only available with a 6.0L but you could get 3.73 or 4.10 rears (the 2500HD was only offered with 4.10 ratio unless you got the 8.1 or the 6.6). The 2500HD was offered with the 6.0, the 8.1 or the 6.6 engines.
The 1500HD has a 4500 lb front axle and 4500 lb front suspension vrs the 4800 front axle and 4500 front suspension in the 2500HD.
The 1500HD has a 6000 lb rear axle and rear suspension vrs the 6900 rear axle and 6084 rear suspension on the 2500HD.
The 1500HD was only offered in a crew cab with a 6.5' box vrs a full line offering for the 2500HD.
The max payload of the 1500HD was 3129 lbs vrs 3606 for a comparable 2500HD. Tow ratings run up to 10xxx lbs."
The 1500HD is a 2500 and will be a step up from the std duty 1500 with the 5.3 engine.
