Proto-Norse had for sure two allophone l sounds, a "back" l (IPA [ɫ]) and a "front" l (IPA [l]) and a "back" l (IPA [ɫ]). The back l was a short consonant (i.e., l) and the front l was long (i.e., ll) in Proto-Norse. The cluster ld was pronounced [ld] with a front l, though.
In Modern Jamtlandic the back l has turned into a retroflex flap [ɽ], let's call it a "thick" l, except in the beginning of words and after front vowels where it has become [l], let's call this a "thin" l. (There are a couple of further rules of exception; see §38 in this document.) The front l has stayed the same.
The question is now, is there any use for a special symbol denoting the thick l? The thin and the thick l's are clearly not allophones anymore in Jamtlandic, and there are no 100% waterproof rules to tell whether an l is thin or thick. A bisyllabic Old Norse word -ila would have a thin l in Modern Jamtlandic due to the front vowel i, and a bisyllabic ON word -ala would have a thick l due to the fact that a act as a back vowel. Due to vowel levelling, the words would only be separated by the quality of the l. This suggests that the thick l needs its own symbol.
What symbols should we use? Preferably a diacritic of l. Polish ł seems to be the given canditate. In Modern Polish it denotes [w], but in older Polish (and still in archaic dialects) it was pronounced [ɫ], i.e., the Old Norse pronounciation of what has become a thick l in Modern Jamtlandic.
To conclude, the thick l in Jamtlandic is written ł.
Needless to say, in situations where one has assimilation of thick l with another consonant giving a retroflex consonant, one still writes out ł rather than using r. Hence, we write e.g. gułd [gɞɖː] neut. 'yellow', not something like "gårdd" (see §38 in this document again).