Rage is not mere anger; it has aspects of violence and abuse. It may have underlying psychological causes.
(Jude 15,16,18,19) To execute judgment upon all, and to convince all that are ungodly among them of all their ungodly deeds which they have ungodly committed, and of all their hard speeches which ungodly sinners have spoken against him. These are murmurers, complainers, walking after their own lusts; and their mouth speaketh great swelling words, having men's persons in admiration because of advantage. How that they told you there should be mockers in the last time, who should walk after their own ungodly lusts. These be they who separate themselves, sensual, having not the Spirit.
It seems that Jude has in mind here people who do not live out Christian virtue. If murmurers and complainers are in this category, certainly those who have extreme rage outbursts are also.
(Galatians 5:19-21) Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, idolatry, witchcraft, hatred, variance [dissension], emulations [jealous rivalry], wrath, strife, seditions [insurrection], heresies, envyings, murders, drunkenness, revellings [boisterous festivities], and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.
The definition of mortal sin is that it results in not entering into heaven. From this verse we see that rage is indeed a mortal sin. The salvation of a Christian who indulges in rage is at risk — we should assume they are not saved just to be on the safe side. Rather than merely think that they are saved by grace because they are a believer they should take this passage to heart.
(Romans 16:17) Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them.
Certainly rage causes division and offences. But Paul may be referring here to bad teaching rather than bad behavior.
(2 Thessalonians 3:6) Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us.
If we are to shun fellow Christians who are merely disorderly, surely we should shun those who have much worse behavior such as those who exhibit rage.
It is possible that Paul is referring to people who are corrupting the gospel, to false teachers and heretics, and not to people's bad behavior. If this is the case then it seems that bad teaching is worse than bad behavior.
(2 Thessalonians 3:14) And if any man obey not our word by this epistle, note that man, and have no company with him, that he may be ashamed.
It is proper to shun people who disobey God's law. However, the purpose for this is that they would repent. It is unclear whether someone can control their rage or not without proper psychological assistance. Perhaps shunning has a role in helping the person see that they need psychological help.
I think it is reasonable to stay away from people who have demonstrated behavior in which they harm others or damage property. If you are unfortunate enough to happen to be in their presence when they have a serious fit of rage, you might become a victim; perhaps physically harmed or accidentally killed. This is not Christian shunning per se but it has the same effect.
Based on the passages above, rage is a mortal sin and Christians are to shun other Christians who habitually practice mortal sin.