My turkey is already cooked, so I won't have any pan drippings. Can I use chicken broth and a thickening agent to make gravy instead?

Definitely. Use can even add a chicken boullion cube to make the taste stronger. I personally always grab the powder turkey gravy mix - not the best for you (flour or corn starch in it) but I only have it occasionally, so I don't worry too much.

You can thicken with egg yolk too. Just stir small amounts of the hot liquid into the yolk before stirring it into the liquid. Another trick is to thicken with veggies - mashed carrots or finely chopped mushrooms (duxelles) works too.

Hmm, never heard of using egg yolk - sounds kinda gross, LOL!

It thickens the sauce as it cooks - old French cooking trick! (You can use blood too but that's really gross!)

Mmmm... it's good thickened with egg. Veloute sauce with egg and cream... oh, yum!!

Just yolk tho, no whites, right?

Yes. Just the yolks.