The limiting factor on towing weight would be the springs. The 3500 has a couple extra for more weight with the same axle, and the dually adds a little more just due to tire load, but same axle. Automatics seem to be a hit or miss on Dodge trannys (use a tranny temp gauge to prevent failure), but the NV4500 5 speed has a common problem with Dodge for losing the nut that holds the 5th gear on. Usually no permanent damage, but you gotta get a locking nut or some people tack weld theirs in place. Mine is a 98' 3500 with the 5 speed, had to do the 5th gear fix, but no problems for the last 15k of towing 9k of load. And it's also my daily driver.