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English Wiktionary and Spanish Wiktionary disagree on the pronunciation of the word, but they both could be reasonably approximated by the English J sound. English Wiktionary:
/ˈɟ͡ʝunke/, [ˈɟ͡ʝũŋke]
ʝuŋ.ke
The difference between the two is that the first is an affricate, while the second is a fricative. This is analogous to the difference between English ledger and leisure. (English doesn't have the corresponding sound at the beginnings of words, or at least not as far as I can think of, but if you know French pronunciation, you will recognize the difference as that between English j of jam and French j of jambon.)
In this case, however, the affricate is palatal, whereas the sound in English is palato-alveolar or post-alveolar; in other words, the English sound made farther forward on the roof of the mouth, the Spanish sound farther back.
The Spanish sound is made on the palate, like the ch in German ich, but unlike that sound, it is voiced. This difference is analogous to the difference between ch in chunk and j in junk.
The article on Caribbean Spanish in (English) Wikipedia notes that one feature is
Yeismo, where /ʎ/ and /ʝ/ merge to /ʝ/, as in many other Spanish dialects.
(The first sign represents the sound that is normally spelled ll in Spanish; this sound is commonly rendered with the English y sound, as in La Jolla, California, the spelling of which I did not connect for many years to the name that I had been hearing as "La Hoya.")
To an English speaker's ear, both of these will sound more or less like the sound of y in yes, because the sounds represented by Spanish ll and y don't exist in English. The English sound is a palatal approximant, which means that the tongue approaches the palate less closely so there is no audible hissing. If you say yes and stress the y by bringing your tongue very close to your palate, you will actually produce the Spanish sound.
So the answer is that the standard Spanish pronunciation shares some features with both English y and English j. It is articulated in the same place as y and it is either a fricative like g in mirage or an affricate like j in junk, and it is articulated in almost the same place. Taking these similarities into account, it is no longer surprising that English speakers sometimes adapt the pronunciation using one English sound and sometimes using the other.