I would assume that Moses and Elijah were dead while Jesus walked the earth. I suppose that they could have been raised before this time but to me that seems less likely than Christ's explanation that what they saw on the Mount of Transfiguration was a vision.
Matthew 17:9 KJV
(9) And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
Hypothetically, if Moses and Elijah had been walking side by side through the streets of Jerusalem, how would anyone have recognized them? You might recognize Abraham Lincoln or George Washington because their faces show on money, but the Jews didn't keep pictures of people at all. With that in mind, look at the verses that come next:
Matthew 17:10-12 KJV
(10) And his disciples asked him, saying, Why then say the scribes that Elias must first come?
(11) And Jesus answered and said unto them, Elias truly shall first come, and restore all things.
(12) But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them.
If they had seen the literal manifestation of Elijah, why are they asking about Elijah like this now? The Elijah that Jesus talks of when he explains to them shows that it was intended to be understood in a symbolic context.
Matthew 17:13 KJV
(13) Then the disciples understood that he spake unto them of John the Baptist.
The context (and Christ's explanation) does point towards this being a symbolic vision, rather than a literal appearance of specific persons. Thus my assumption that Moses and Elijah remained dead and buried during this time, rather than being specifically raised for a cameo appearance.
I can understand the thought process you present about 'visions', and it is something to consider.
Usually when a person in scripture is witnessing a 'vision' they are in sleep or trance-like state of being.
I don't see that as being the case when the disciples saw Jesus with Moses and Elijah.
It is written as if they are fully awake and aware, even to the point of being worried if they should build a tent for them.
And they hear the voice of the LORD and are so afraid that they fell on their faces.
None of this seems to imply that they were asleep or in a trance when this took place.
What type of body they had when the disciples saw them is unknown.
It is a good assumption to think their old flesh bodies were still in the grave, as was David's.
It is my assumption that what the disciples saw was their spirits, and not their resurrected bodies.
One of those rare instances when the human eye can see the spiritual realm, as when Elijah saw the army of angels all around him and asked GOD to open the eyes of his servant so he could see them also. 2 Kings 6:17
It was really happening, and was not just a dream.
But I leave my options open for further study.
I have seen or heard many types of explanation as to the importance of this story, and what it's intended message revealed.
They seem incomplete to me, as if there is something more to the story.
I don't think John the Baptist being spoken of as Elijah has anything to do with this story.
As he is said to have come in the spirit of Elijah. Luke 1:17
Good study!
Thanks for your responses.