I needed a 6" drop shank for my Ram 2500, with a 9500 lbs trailer. Remember that your 2500 won't sag nearly as much as your half ton did. I think I drop an inch or so when I hook up, but I don't remember exactly.
The 6.4 should do fine with your trailer. My friend had a 6.4 and it pulled a slightly heavier 5th wheel. His biggest complaint was the gearing. Its a gas engine, its going to rev to get in the power band, but the spacing of the gears annoyed him. He'd approach an incline and the truck would downshift to 4th and try to hold speed, then eventually drop into 2nd (completely skipping 3rd gear) and the RPMs would race. He would usually stay in 2nd gear to the top of the hill before it finally started to shift again.
I'm sure this could be mitigated by "manually" shifting into 3rd if you approach an incline though. Other than that, its a great truck. The gas is super quiet and creates a lot of power. Its substantially less expensive than the CTD, but its pretty capable, especially at those lower weights you are towing. I'm sure you'll be quite pleased once you figure out the quirks of the transmission.
2015 Ram CTD
2015 Jayco 29QBS