Since you apparently like a GM SUV I would simply upgrade to a 2500 Suburban (or Yukon XL). The standard 6.0L engine will be a nice upgrade. I would want 3.73 gears for your trailer weight. If you really want some power find one with the 8.1L big block. These were made from 2001 to 2007.
There are companies that will build for you a 2500 suburban with a Duramax diesel. Duraburb is one such company, located in Florida. I just say one of these creations Sunday pulling a boat at the marina. I asked the owner and he said a guy in Ohio built it for him.