Dealers will sell individual parts, but some do not only because the profit margin isn't worth the stocking cost. For future reference you can visit RockAuto Auto Parts
Ford and Dodge use snap rings to hold the joints on. GM uses urethane (last I checked), which makes removal easy - just torch them out but durability is questionable in the long term.
There were no significant driveline changes from the 2006-2007. That body style stretches from 2001 to 2007 where most changes were cosmetic. As for the VIN, most vehicles are made the year before to sell as next years model. Some January/Feb built vehicles in say 2007, might sell as a 2007 model and one built in March might sell as a 2008. It depends on the factory and how quickly they can retool any model changes.
The 2008-2012 models are different in several ways, mostly style although the powertrains were tweaked and the base engine gained displacement.
If the vehicle runs fine with the new U joint in place, there shouldn't be any worry about mechanical problems from this down the road. Just keep up with fluid changes in the pumpkin and periodically inspect the underbody. It was in 2008 when the electrical 4wd system was upgraded to react to changing road conditions to within fractions of a second. If a drive component is loose this can trigger not only dangerous driving conditions in poor weather, but can have adverse effects on the braking, powertrain, electrical, and vehicle as a whole. I'm glad they didn't go all out and get an AWD system like Audi's Quattro - something no one wants to deal with after 3 years.