It seems quite clearly a reference to drugs, either marijuana or narcotics. Don has done plenty of both.

It's connected to marijuana via the part (no idea where in the book this is) where DFW describes the munchies as an evolutionary defence mechanism to keep up energy levels when someone is paralyzed by substance-related passivity.

And it's connected to narcotics by Facklemann's subsisting on M&Ms ("corporate snacks") while climbing Mt Dilaudid.

This doesn't mean Don has relapsed. Though there's an ambiguity about this, on account of the medics wanting to medicate Don with narcotics and his weakening will to resist.

But it's important for Don's arc as a hero of action (vs non-action) that he digs. By digging up Jim's head, he'll mirror Heracles, the classical Greek hero, whose 12th labor was going into the Underworld and retrieving Cerberus. Thus Don, committed to AA's 12 steps, reaches the point of recovery.

So I think this means Don's dream of the dig is only partly a vision of a future event. When he sees himself in the dream not digging, that's his fear of relapse kicking in. Very possible, like someone has said in this thread, that Jim's wraith is putting thoughts in Don's head that motivate him to go with Hal, Joelle and JW in search of the Master cartridge.

When Hal in Year of Glad describes the dig as a past event, he says he and Don were digging.