I think it's important to remember that the term "Adivasi" doesn't really mean anything outside of politics. If I'm remembering correctly, "Adivasi" doesn't appear--in print or in popular discourse--until several decades before Independence. The term was, in effect and essence, coined to connote a political identity for India's disparate tribal groups, who have little in common besides what has been termed a "share history of oppression," displacement, or general marginalization.

And that makes sense, if you spend a second thinking about it. There are significant social and political cleavages between Gonds in relatively small sections of Chhattisgarh, let alone between Gonds in Narayanpur and Tadvi Bhils in Gujarat.

However, a great many Adivasis, especially in particularly rural areas, still abide by local traditions and practices. Oftentimes, these practices have some overlap with mainstream Hinduism, or incorporate similar rituals, deities, or stories. I suppose you could consider them proto- or quasi-Hindu, insofar as some recognize or pay homage to classical figures, such as Shiva and Ravana, while emphasizing the comparably active role of village gods (or goddesses) and nature-based spiritual forces.

Not an expert on the subject, by any means, but Adivasi religious affiliations--and alliances--are actually a pretty interesting topic. I recall reading a dissertation that provided a pretty unpleasant overview of how RSS activism facilitated land conflict and communal strife between "Hindu" Adivasis and "Christian" Adivasis in rural Chhattisgarh. Can't remember the specific details, but someone dreamed up a Christian conspiracy that centered around mahua-liquor proceeds being used to steal "Hindu" land, which was then force forcibly appropriated.