my voice get's really shaky when I play a mid-tempo song using down-strums.
I can handle normal strumming but the extra effort I put in my 8th note down-strumming is causing my voice to shake.

any tips how I can get passed this?
I would think using less arm and more wrist when you strum would help, since it's the jerking your arm around that's causing your 'shaky voice.'
Strum up and down instead of just down.

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It may be that you aren't completely used to the song your playing yet. When I sing and play guitar at the same time, I need to have the song completly masterd, and not have to think about the playing at all in order to sing my best.
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Happens to everyone. It's hard to play complex, or at least contrasting rhythms against your vocals, and in some situations impossible depending on the musician.

My advice is first, play the song holding the chords for their duration instead of strumming while singing. Second, play the guitar while either humming/talking/whispering the lyrics. Keep alternating between the two methods, each time trying to incorporate more elements of the song until either way sounds the same. Then just keep practicing to improve.

A lot of people's problems is they concentrate to hard on either task. Most people can get away with singing over a simple chord progression, but when the rhythms get a little more complex, or at least demanding of you, you concentration strays from your vocals, and you probably forget to breath with your diaphragm or fail to get enough air someway, which makes your voice 'shaky'.

You have to think of the rhythm of both the guitar and your voice as overlapping, recognize what happens to one while being aware of what you're doing with the other. If you have to get a score of the song with the vocal melody, use guitar pro or something, and have a visual cue of such. Good luck
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