The back tailgate will not go down since one of the side locks will not release. Any suggestions?

Well, yep, there is a lock on both sides, each one has a rod going to if from the central handle. 1st thing is try to push in on the tailgate while pulling up on the handle as much as possible. Don't be afraid to throw your weight into the tailgate, just use your shoulder, you wont dent these good ole heavy metal goodies. Sometimes they just get out of adjustment (i had to do that trick with a few door handles to get the doors open). Luck or no luck with that trick, you have to take the inside tailgate panel off, first all the metal strips, then the carpet and then you will see a large access panel in the tailgate you will have to take off. Don't worry, it all comes right off even with the tailgate closed. NOW CAREFULLY, you will need the rear window up to be able to reach inside and unlatch the stuck side, you may just be able to pull on the rod, look at the other working side to see how it works, or if the rod came off right at the latch, you may need a long screwdriver to reach it. Now only unlatch the stuck side. And yes once you get in there, you want to lube every single part in the tailgate with a grease like white lithium grease (wd-40) is not a lasting lubricant like grease. Once in there, you should be able to see either how to adjust the rods, usually threads at the ends or if one of the clips have broken.