The 2006 and early 2007 Model Year Duramax's are the fastest moving used models off the dealers lot. Some of us, me included, looked for months to find a low mileage LBZ powered GM. The transmission is the model that all other manufacturers want to emulate...and for the most part they have succeeded in later model years.
The LLY is basically an under programmed LBZ. Same engine with less horsepower...and maybe only five of the six speeds are programmed to engage. These limitations can be easily changed along with adding a quite nice exhaust brake. The market is full of solutions to accomplish this.
One only has to look at selling prices for these Gold Standard model year trucks to verify the statements made in this thread. I have put over 30000 mile on my low mileage 2006 LBZ and it is still worth more than I paid for it.
Have fun...and if you like that truck...buy it quickly before it is sold to someone who did not hesitate...BTDT...
Regards