was the "Medieval Ranger" archetype extant prior to the writings of J.R.R. Tolkien?
Yes.
Tokien's avowed goal was to create an "English Mythology" that would be comparable to works like the Kalevala. The intellectual tradition that he belongs to is that of 19th century historical linguistics (philology), which saw the study of language as not just a technical study of logic and grammar but as the study of the development of cultures and literary traditions. It was scholars like him who, in the nineteenth century, "rescued" folk tales and shaped the emerging national languages of Europe.
Given this, Tolkien was not creating tropes out of whole cloth. He was consolidating and integrating them into a consistent story cycle. In my view, the best way to think of the LotR is as taking fairy tale tropes (goblins, demons, fairy queens, princes trying to reallise their kingdoms etc) but presenting them within the framework of the naturalistic novel; Tolkien knew from experience that there was a limit to how seriously modern audiences would take non-naturalistic, non-novelistic stories (eg Beowulf, Gawaine and the Green Knight).
In Tolkien's story cycle there are a number of rangers: Turin Turambar (a human prince, fostered by an elven king, whose unhappy life ends in tragedy); Aragorn (a human prince, fostered by an elven king, whose heroic life results in him gaining his kingship and wedding the elven king's daughter); Faramir (a human prince, mentored by a wizard who is close to the elves, who is separated tragically from his flawed father but in the end comes into his inheritance in part because his wise mentoring allows him to understand the true nature of his inheritance as a steward and not a king). (There are also the various bit parts, like the sons of Elrond, the rangers who join Aragorn and ride with him through the paths of the dead, Faramir's offsiders like Mablung et al.)
The idea of a prince having to go into the wilderness to be fostered before coming into his inheritance is not invented by Tolkien. Romulus and Remus (fostered by a wolf) provide one example. Siegfried in Wagner's treatment of the Nibelungenlied (fostered by a dwarf) provides another.
And there is the most famous English example:
Robin Hood is a big influence.
is there evidence out there, possibly an interview or autobiographical work, citing Tolkien as having been influenced specifically by Robin Hood when creating the ranger?
I think it's obvious.
Robin Hood is the most famous of English woodsmen. There are versions of the story which identify him as himself a noble in hiding, or waiting to come into his true inheritance; and other elements of Robin Hood tales include his loyalty to the "true king" (Richard the Lionheart). Robin Hood doesn't have the fairy-tale dimension, however - he takes refuge in the wilderness but is not fostered by it.
I think the aspect of the ranger archetype that is unique to Tolkien, and hence whose presence in D&D is attributable solely to Tolkien, is the idea that they are true descendants of the "men of the West" and hence able to claim and use the Palantiri. Though when this appears in AD&D as the ability to use crystal balls once reaching 10th level, some of the mythological resonance has been lost!