I am shopping for a used 'burb and would like some suggestions. The vehicles I am looking at are 2002 and older; the oldest is a 1998; the lowest mileage is 180K, the highest 220K. The bodies and interiors of all look really good, almost brand new and everything is functional. Engine choices are 5.3, 6.0 and 7.4. In the next couple of years I will be using this vehicle to move things cross country; after that, it will be used locally, but still for towing a small to medium sized trailer. An idea I had, when the engine gets to be too tired, is to take the engine out and have a diesel installed. Is that practical? I am leaning to a 7.4 because it is already set up with a heavy duty suspension - might be easier to put a diesel in it? Is the 7.4 or 8.1 the more durable choice? What are the things I should look for, shake, bounce, thump when looking at these vehicles? What are the weaknesses? The requirements I have are that it has at least front airbags, is 3/4 ton, has good towing capability and AWD