Any head-tank (or tank role-lead or what have you) that treats his co-tanks (MT/OT is dead imo, you use the best key to open the way to the lewt) like sh*t doesn't realize that in 25 mans - very rarely can content be solo-tanked, and won't learn that lesson until they are the only one left - or they are thrown out in the cold themselves.
Speaking as the tank that is usually thrown at the highest content we can achieve so far - I realize every day that I get the glory for the hard work done by others in the shadows. I also try my best to not disable them by taking major upgrades away from them just because it's a minor for me. I shift the focus on them, and I will make sure they can have the spotlight if they send me a tell saying they want to give <boss> a shot. I'm not the best skillwise, and I'm about tied gear-wise. However, I think they keep me around because I'm willing to toss the reigns over now and again so that I can share the lumps, bumps, scrapes, and repair bills.
When raiding - you're only as strong as the weakest link.
Put me down as a vote for "move on." I really don't work well with ignorant people, and don't think anyone else should suffer them either.
If the guild isn't suiting your needs, why stay? There's no reason why you can't keep in contact with your friends in the guild; one person I know created a chat channel as more and more of her friends left the guild she is currently in (long story as to why she still stays); everyone just joins that channel and they're still able to hang out and group together for heroics and raids.
Being sidelined from being MT because you know your limitations, know that you need to be better geared, know that everyone else needs to be better geared is pretty shoddy of the GM IMO. You have to be strict right now with who you let in for Ulduar, it's not just something that just anyone can do when they haven't gotten at least gear from 10 man Naxx. And doing all of this just to appease one person? Putting one person above the rest, even if it is the MT, doesn't really seem to lead to the rest of the guildies feeling like they're valued contributors, especially if some of them might feel similarly to you in terms of what they know they are ready for.
I think that you're probably wiser than your higher ups in your guild just for realizing that it takes a lot more for Ulduar than it does for Naxx, and that should be commended, rather than having you unable to adequately progress with your guild because they have ilevel 219+ eyes. If you have to ask if it's time to move on, chances are that it's been time to move on for a while now.
Think about this; content progression and guilds require leadership. I understand your frustration and I would submit you that you undertake a project of organizing your guild to accomplish some 10 man content with you as the raid leader. In this sense you cheive 2 things, you can learn Naxx easier with the normal mode of Naxx and hopefully use a dps from the heroic group that knows the content and mechanics and get the loot drops that will allow your 10 man to progress into Heroic raids in Naxx.
I dont think quiting the guild is the answer, leadership is. In a void of guild leadership provide your own. People in the guild will follow a tank into progression if your prepare and know the fights, ask people to come prepared with pots, food and a good attitude. Learn about raid composition and what you will need to successfully complete the content. Be loot master and follow the loot rules of the guild and never sacrafice you integrity for a piece of loot.
Take the 80's, and get your own Naxx team started. They want to raid and get geared up, and they probably need someone to lead them into Naxx. If you're no longer comfortable in your guild or your guild is being a pain about it, you might want to consider taking as many of those 80s and starting your own guild.
Take what remains of the group and start running Naxx for gear. Try to do Naxx 10 and also Naxx 25 and invite friends/randoms for it. Start being the raid leader for these raids, always keep a smile and a positive demeanor.
Set the ground rules for any newcomer(s): enchanted gear, gemmed gear, flasks and stat foods on at all times. If they follow that they are eligable for all loot. make sure you set up the loot rules simple and proper.
While you do this you make sure to study each and every single Ulduar fight as much as you can. Then lead your team into Ulduar and catch up with the other team.
Sooner or later you will be asked into the "main" team. Tell them that you cant because you have a responsibility to the other team, but you will gladly come along on the big raids if you can share the tanking in Ulduar 25'