Is there a mod out there that will change more than just one bar when I shift into a form? I've got all sorts of powershift macros in cat that I don't need in other forms, and no place to put them on my bars (and bind them on my keyboard), not to mention downranked healing and other tools that I only need in caster form.

Trinity Bars has the best complex bar handling I've seen in a mod.

I have mine set up to:
Paged in caster / moonkin / tree (obviously spec-dependent) but not page in feral
Switch multiple bars with form changes
Stealth bar
All of the above having modifier keys that change what they do, all varying again by the base state.

Some bars are hidden out of combat, others only visible on mouseover, others with alpha changes on cooldown.

The bars also cascade/stack (I forget the term Maul uses), so if I have an alt-modifier for one button but the one next to it is unmodified it continues to use the unmodified keypress for that one regardless of the alt key's state.

Granted it took me 45 minutes to set up that way, but I don't think I'd be able to get the flexibility out of another bar mod no matter the time I put into it. Don't give up immediately if you do try them out. It had a bit of a learning curve for me. I think of it as the Grid of barmods. Just prettier.

Bartender 3. All the functionality of trinity and about 5mins to set up with experience maybe 20 your first time.

I have 2 bars that switch based on Bear, Cat, Moonkin, Tauren forms and a 3rd bar which switches based on Taruen or Moonkin for clickies.

I use autobar for tidy consumable buttons.

Another bar mod a lot of people (that I know) use is Bongos2. It provides a bit more than just your vanilla bar mod in that it also allows you to move and configure the mini map, the cast bar, the roll pane (where loot pops up when the loot setting is group loot), etc. Configuration is per bar/pane and has configuration panels instead of menus.

I have been using Bongos for a while now and have enjoyed it but I know a lot of people use bartender3 in our guild (specifically Warriors for stance changing) and speak highly of it as well. From a purist standpoint Bongos tries, IMO, to do a bit too much. Each piece is a separate unit and they can be enabled and disabled (or excluded entirely) as you see fit - but it must be done from the Addons menu at the character select screen.

Bongos and Bartender are both good choices (I will be switching to Bartender soon).

There was a problem for a day or so following the 2.1 patch where the bars would get locked if you were in combat, making the mod virtually unusable for a druid (and probably for others, too), but that was fixed within a matter of hours (I had to tank Anzu using my spellbook for my bear abilities, which I don't recommend). BT3's been rock-solid for me, otherwise. I've certainly never had the bars fall out-of-sync due to rapid shifting.

so I feel dumb, but I've gotta rectify my earlier posts on this matter. I use my scroll wheel to shapeshift, and after using trinity for so long, I'd forgotten that shift + scroll wheel changes pages (I've since unbound this). I was looking at my keybinds and noticed this, so I reloaded bartender and messed with it a bit and I think that is what was happening - I was shifting quickly in combination with shift + button keybinds and messed my whole shit up. that being said, trinity is still far more configurable, but bartender is a strong competitor.