From what you have told me, I suspect that it has to do with the construction of jambs (that is, columns, pillars, wedges, or other supports) that may have been used to build structures within mines for the safety of miners, for transporting ore, and the like. For a photograph of such a structure from a California mine, see the website listed below. [1]

Just one look at the photograph gives an idea of the tough work that would be involved (especially in the outback of Australia) in supplying the timber, sawing and trimming it, and erecting the structures in the confines of an underground mine.

Jambs, as you know, now usually refer to the vertical supports of doorways, as well as mantelpieces, window frames, piers, and other frames requiring strong support. The word is derived from Old French "jambe" and Latin "gamba," refering to a horse’s leg or hock.

However, as a term specifically related to mining, jamb also referred to "any thick mass of rock which prevents miners from following the lode or vein." [2, from 1913] Among lead miners, it could be "a thick bed of stone which hinders them when pursuing the veins of ore." [3, from 1928]

Hence, to gold miners, builders’ jambs would have been specifically designated as "wood jambs" to distinguish them from the natural stone jambs in the mines themselves.

Building these wood jambs, or frameworks, therefore, would be "wood jambing." This would have required different skills and tools from other mining tasks; hence, it would have been a trade or job in itself, as you indicate.

Without reading the story, of course, I cannot be sure that this definition exactly fits the context. But I think it’s in the ballpark, at least. For a thorough study of the word, its definitions and etymologies, check the 28 dictionaries linked to this website. [4]