Where did you get a 1500 lbs curb weight? That sounds more like a net load rating or something like that. My '95 was a curb weight of 6300 lbs.
the truck was brought in from out of state as a salvage vehicle and the previous owner had it registered that way. not sure how the DMV let him get away with it for so long, but the weight fee component was $8. Its now over $200 with the proper curb weight.
I only noticed it when I paid my 97 Ram gasser registration the month prior and the 99 CTD was way cheaper. Last year my wife paid one bill and I paid the other, so we didn't catch it. The previous year was the first I owned it and just did a basic title change and the fees were up to date, so I didnt notice then.