I believe it has to do with the GVW rating on the pickup. I never had to do emissions on my Ram 2500, although I did pay quite a bit extra in registration fees and taxes. Lower GVW vehicles, diesel or gas, gets a ODB reader check here. They recently lowered the weight limit, don't know what it is off hand.
Current: 2011 Grand Cherokee Overland V8, 2009 Liberty Rocky Mt V6
Previous: 2000 Grand Cherokee Laredo I6, 1979 CJ7 I6 Quadratrac