Actually the first step would be to go to all devices and set your sync options to Manual, then turn off your MobileMe sync on all devices for now, to make sure you don't get bad data syncing. On one machine then perform the maintenance on it to get Keychain right.

After getting it right, go to MM preferences on the fixed machine, turn on Sync. Click the Advanced button on the bottom right. Then click the Reset Sync Data button. Tell it to replace what's On MM with sync info from this computer, Replace.

Before doing the Replace make sure everything, Keychain and whatever else you sync is what you want.

Then go to the other machines, do the same as above in the second paragraph except choose to sync data On this computer with sync info from MM.

After that you can then go to each machine and set it to sync automatically or whatever interval you want.