Absolutely EVERYTHING Gina said -- have them pay particular attention to teeth, thyroid (I'd have them send it out to Michigan State -- they are SUPER good with things that influence thyroid like age, medication, etc.) and anything else -- it's likely some *thing* that is causing him to be excessively sensitive.
Also -- everything Gina said about pack order -- you may have to stop and sit back and re-examine how things are handled -- keep a journal on this -- go back over the situations as they've happened -- what position is everyone in, what time of day is it, what was laying around (toys, food, family just 'chilling', or whatever). What was Legend doing -- sleeping? WHERE? his own bed? next to a family member? etc. What changes have occurred? any newbies? any human schedule changes, etc.??
You may just plain have to keep these two dogs completely and utterly separated -- permanently.
We recently had to change a crating situation -- I had both Luna AND Billy getting more than a bit annoyed at Kee Shu -- she's as dim as they come, frankly. She's sweet, but she can get REALLY obsessive/compulsive about things and I saw both dogs, at completely different situations/scenarios get growly with her. She's so much smaller that I dared not leave things as they were.
She's always hated to be crated alone -- before she came to me she lost her brother that she'd been with all her life. she WANTS to be with another dog, particularly with a male -- she WANTS to *not* be alone. So I'd always let her be in with Billy ... and then later with Luna.
Frankly, both of them got fed up with her -- they never hurt her, but I could tell it was TIME FOR A CHANGE so we got back out the smaller crate for her and that was the end of the nonsense.
She was driving them nuts while we were gone ... and they did marvelously at not erupting at the time, but it had them both edgy and then when the slightest provocation occurred it push the envelope. She's gotten used to being alone (they're all in the same room) and she's fine with it and the other two have resumed happy happy relations with everbuddy. IN fact, I now see them both take time occasionally to lie down next to her. It's as if they *know* she needs that contact ...
So it may actually have its roots in something completely different -- but think it thru and see if you can identify a time when things go awry.