I've heard the Vikings were descendents of the Hebrew Tribe of Dan. Is it true?
In short: no.
Culturally, the Vikings are well-documented to be part of the Norse culture of the Dark Ages, which in turn is clearly descended from an earlier common Germanic culture.
Linguistically, the Vikings spoke Old Norse, which is part of the North Germanic branch of Indo-European. The Israelites spoke Ancient Hebrew, which is part of the Semitic branch of the Afro-Asiatic language family. If the Vikings had been an Israelite tribe, you'd expect them to speak Hebrew, or have Hebrew borrowings in their language (which they don't). Israelites picking up a Germanic language would be expected to end up with something more like Yiddish, which is riddled with Hebrew influence.
Genetically, there's some very interesting material to look at about Y-chromosomal haplogroups. Jewish communities worldwide have a high proportion of certain haplogroups (such as the famous J1c3, common among Kohen families). Scandinavia (where the Vikings lived) has an extremely low proportion of those haplogroups. Here's a Wikipedia article on haplogroup J1 to get you started.
The website you linked to makes some interesting claims, but they don't link them together very well. For example, they make a claim that the tribe of Dan might not have been Israelite at all, and then they jump straight into assuming that the tribe was connected to Tarshish, without any evidence of that. Later, they identify the tribe of Dan with the legendary Tuatha de Danann without any evidence other than having vaguely similar names. Then, they claim that the Scythians were Israelites, based solely on the Jewish holiday called Sukkot. Here's a great article from Mark Rosenfelder explaining this phenomenon: How Likely are Chance Resemblances between Languages?.
The Jewish diaspora did manage to make its way to some fairly far off lands - India, Ethiopia, Spain... there is a written historical record, and also strong archaeological evidence to document their migration to these places.
Unfortunately, there is no such record or body of evidence to support Scandinavia as one of those far off lands. 15th century musings on the topic are political in nature - Leibniz was trying to lay down a historical justification for the re-unification of the Church. 19th century interest is due largely to a large upwelling of public interest in mysticism, secret societies and "hidden knowledge" (much like today, with our Ancient Aliens documentaries and ghost hunting shows on cable), and not credible evidence.