How to I shift my dialogue into narrative or descriptive text and still maintain the character development, relationship development, and plot movement that I get from writing dialogue?
You don't. If you can do character/relationship development done through dialogue, that's excellent. Note frequently shifting to non-verbal communication will have a better impact, but that's still a dialogue, only without words. But if you can shift descriptions from narrative into dialogue without loss of detail, by all means, do! It's easier to read and more interesting - but harder to write.
If you feel speech doesn't convey something well, only then shift into descriptions and observations. It's easier to say "He wore his usual, worn, sleeveless jeans jacket with a thousand patches" than to think up an excuse to make the protagonist talk with someone about their jacket, but if you can think up a viable excuse and the dialogue will be interesting, choose dialogue!
Use narrative for action. When all is said, there remain things to be done. Yes, plot movement. When words fail to solve problems, let the actions in, and skimp on dialogue not to distract the reader from things that happen.
If the action is not catchy enough to stand on its own, you may overlay dialogue on top of it. Characters perform while talking.
And from time to time, when the action stops, and when most is said, use a section that is neither action nor dialogue. Break the pacing with a reflective, descriptive, emotional, introspective, calm and slow piece with no action whatsoever, that is practically a shock compared to the remaining whirlwind.
How do I determine whether there is enough dialogue, or too much?
Don't worry about that. It's as if a painter was worrying they use too much brush#4. That's unimportant. Worry about pacing, about build-up, about mood, immersion, coherence.
There are stories that are best written without one single line of narrative. The dialogue builds the place, the mood, the conflict and the resolution, and the lack of narrative gives an eerie immersive feel of listening in to actual conversation. And there are stories where words are best avoided. Painted by rich descriptions, poignant in imaging scenes where words would be cheap and shallow. It's up to you to decide which image you want to create and the use of dialogue and narrative is just tools to achieve this image.