I've been looking at 4wd Tradesman crew cab short boxes. On the 2500s, I've found that the payloads run just under 2400 with the CTD and just under 3300 with the 6.4. I assume the difference is the heavier motor. A 3500 CTD generally comes in at a little under 4000 lbs. Granted I am looking at Tradesman trucks which are going to tend to have higher ratings cause they have have fewer options weighing them down.
If you Google Ram Towing the first result is a webpage where you can plug in the VIN of any Ram you're looking at and it will pull up the ratings for that specific vehicle as equipped.