Since it's been about 2 weeks since you posted your question you may well have already done a factory reset on your phone to resolve your problem. These phones "leak" internal storage space on an average of 2 to 5 MB per day (though sometimes it happens in large chunks), no matter what apps you have installed, what email accounts you use or don't use, or what messages you send. The "leak" has nothing to do with how many messages or emails you send or receive, so that won't affect it. I've tried not even installing any 3rd party email accounts, not texting, not using FB, not using news/RSS feeds, etc. etc.. Nothing stops this phone from losing space RAPIDLY from the moment you reset it or activate it.
Sure as the sun rises, if you own an Evo Shift it WILL run out of space within 3 to 6 months MAX, even if you just unwrap it, activate it, and don't touch for that time period. Wiping each of the caches is irrelevant, and so is moving your apps to the SD card. That will buy you a bit more time of course, and obviously the more apps you install the sooner you will run out of space, but you WILL run out of space in a fairly short period of time no matter WHAT do or don't do with this phone, PERIOD. There's a bug in the software or firmware that causes the space to become unavailable and unrecoverable by ANY means.
There's only one solution, aside from screaming at Sprint and demanding that they exchange your phone for anything OTHER than any Evo Shift, and that's to do a FACTORY RESET (Settings -> SD & Phone Storage -> Factory Data Reset). Doing so will wipe out EVERYTHING on your phone, and will bring it revert it back to the first day you got it, which obviously sucks (except system software/firmware updates will remain intact, not app updates though). However, you can minimize the pain by doing all of the following: back up all of your text msgs to the SD card using one of many free apps, back up your list of installed apps using one of many free apps (or even the apps themselves using Astro File Manager), and make sure that all of your contacts are stored as Google contacts, NOT Phone contacts. If your contacts are all stored as Google contacts, then they will automatically be available as soon as you add your Google account to your phone when you reset it (this is a mandatory step anytime you get a new Android phone or do a factory reset, they all require a Google account).
Once you reset your phone, all you have to do is go to the market and install the app that you used for backing up your text msgs, and the app you used for backing up your apps. With those two things you can restore all of your text msgs and your apps. You'll have to download all of the apps again unless you used Astro File Manager to back up the apps themselves to your SD card, in which case you can reinstall them using Astro from your SD card, and it's very fast. If you used an app that only backs up your LIST of apps, make sure you've re-added your WiFi connection and turned it on when you reinstall all of them to save a tremendous amount of time.
Those are the major steps/issues. Obviously you'll lose your progress in any games you were playing, and you'll lose your settings/configuration for most if not all of your apps and games, and will need to reconfigure them. you'll also have to re-customize your menu screens, as there's no way to save them to the SD card and restore them that I'm aware of (perhaps there is and I don't know about it).
It will only take you an hour or two to do the factory reset and get your phone back to about 90% the way it was before. What takes a bit longer is configuring each of your apps back to the way you liked it, and some apps are worse than others obviously. All in all the whole process should take about 2-4 hours total depending on how many apps you have, but you'll be able to get your text msgs and contacts restored and start texting/calling within 30 mins from the time you start.