Agreed.
Religion won't help the case for a homogeneous group of self-identified proto-Germans. There's really no reason to believe they would worship the same gods the same way or even agree on certain issues of theology that other tribes or groups believed. It's the rule rather than the exception to have various different ways to worship the same god, or to prefer one god over the other. Religion could cause conflict even if it is similar. Catholics worship the same deity and have the same very basic belief in Jesus as Protestants, but the differences are such that they are almost completely irredeemable in the eyes of some on both sides. Muslims, too, worship the same god, yet often you hear of one radical group of one Muslim sect killing another. In shinto, there is a general understanding of what the gods are, but it's not uncommon for one shinto shrine or another to claim that it's better to stick with preferring to worship a single god than several. In the end, people with similar faiths that disagree on one thing or another are as likely to find common ground as they are casus belli.
As for similar language, there's no reason to believe that was all that similar either. Before the advent of modern, standardized languages, local dialects could sometimes vary so much from region to region... even from town to town, as to not be mutually intelligible. Look at Oscan, for example. Compare it with Latin. Is it similar? In some ways, yes... But I can't understand it in sentence form without hitting the vocab cards HARD. Similar as they may have been... linguistically, religiously, culturally and all that... The Samnites absolutely hated the Romans, and the feeling was very much mutual. Do you think they would have identified themselves as being "fellow Italians" in a cultural sense? I don't. I doubt they'd want to think of Romans as fellow... anything. If anything, the Romans tried to distance themselves genetically from the other Italic tribes via the Aeneas myth. Similarly, it's worth noting that Aetolians and Macedonians were culturally and geographically Greek (in b4 "ZOMG!! MACEDONIANS WEREN'T GREEK!!", I disagree but that is entirely irrelevant to this), but were both considered "barbarians" by other Greeks at one time or another.
Either way, even if Italians did think of each other as fellow Italians and Greeks did see each other as fellow Greeks, and everything was all happy in the world... the Germans wouldn't have had such neat, clear geographic to contain this concept. The Italians and Greeks had one thing in common - they both lived on their respective peninsulae, relatively isolated and protected by the Mediterranean on both sides. So, what of the people on the Gaulish border? Certainly there would have been a certain exchange of ideas, language and culture, along with the exchange of goods and coin. Skot noted that Sweboz meant "true blood", and was a good word for the Germans... Assuming that is the meaning, since he did seem unsure, it appears to me as if the intent was to differentiate themselves from their perceived differences with other tribes, rather than relate to them. It sounds pretty exclusive to name your group of tribes the "true blood". I'll readily admit that I'm not particularly knowledgeable on the subject of the early Germans, but I can't help but take note of these things.