Together with the references in this post I add a fourth one according to:
 Hammerdalsmålet, Vidar Reinhammar
 Klövsjöord, Gösta Edlund et al
 Orlboka - Ordbok över jamskan, Bo Oscarsson
 Åremålet, Anna-Lena Forsåker
Reference , a dictionary for the Åre dialect spoken in western Jämtland, will provide us with a third (and final?) basis element of the space of Jamtlandic dialects. (Sorry for the math jargon.)
I would like to analyze the spelling for the Jamtlandic word for 'hew, cut'. As always, we take a look at the Old Norse word in order to understand how to spell: hǫggva [hɒɡːu̯ɑ]. Even East Norse dialects seem to have u-umlaut for this word (Swedish has hugga rather than "hagga", see this site.) In Early Middle Jamtlandic (c.a 1350), the conjugation of the verb was in first person probably the following (inf.―pres.―imp.―past part.):
(See here for how this may have evolved through analogisms etc.) In Hammerdal dialect (H), Klövsjö dialect (K) and Åre dialect (Å), the conjugation above has become (ref. [1,2,4]:
This looks like a mess, but we'll try to sort things out. The pronounciations above are (naïvely) consistent with the spellings
The first observation is that the alternative "hogge" (inf.) and "hogg" (pres.) with "o" instead of á is not possible. We also observe that hagg- → hágg- has occured through closing (and a less interesting rounding) of the vowel due to the gg consonant which kind of resembles [w]. (In the article Overlange stavingar i nordisk by Helge Sandøy in Nordiska dialektstudier, see this earlier post, it's clearly proven that there can have been no lengthening of the vowel before the closing.)
When it comes to the imperfect, we see that hugg clearly comes from an older hjugg. One probably doesn't have a dropped j, but rather an intermediate stage jugg in which one has replaced j with h through analogy with all other conjugations of the verb. The form hógg in the Åre dialect requires special attention. It may have been developed from hjó through first jó, then hógg through analogy with all other conjugations (first replace j with h, then add -gg in the end). The problem is that this probably isn't possible since one would have expected an intermediate form hjœ [hɪ̯øː], which would have become "høgg" after the analogical development. Hence, hógg must be derived from hjugg, and it's probably due to a closing phenomenon with hugg → [hoɡː] as intermediate stages. Closing of [o] produces a desired [ʊ].
Finally, let's look at the past participle. Genuine Jamtlandic must have a softening here, i.e., -ggjeð rather than "-ggeð". (I am surprised both Klövsjö dialect and Åre dialect lack softening in this case. probably an analogism with the other conjugations.) In the Hammerdal dialect, a short y is often [ø] rather than an expected [ʏ], so we have to choose between hyggjeð with i-umlaut and huggjeð without. I am pretty confident that Klövsjö dialect u for this word is an analogism with the imperfect rather than an archaism. The i-umlaut is employed in most Jamtlandic dialects in the past participle of strong verbs, so this is indeed a trademark of Jamtlandic. Thus, hyggjeð is the correct spelling.
To conclude, the conjugation of the Jamtlandic word
for 'cut, hew' is
Interestingly, we observe that the Hammerdal dialect is, among the three dialects studied, the most consistent with the Jamtlandic orthography in this case. It feels like this often is the case; it's possible that the Hammerdal dialect spoken in northeastern Jämtland is one of the most archaic dialects spoken in Jämtland.