I would try aligning the rear wheel using the swingarm pivot points as reference. If that doesn't do it, take the wheel in and have it rebalanced. In that order just because the alignment is easiest and free, tho balancing would be the more likely of the 2. If you are sure the tire and pressure are ok those are what i'd look at next before going on to more tedious, costly and less likely things. remember.....the second you hop on the bike after a tire balance it begins to gradually lose balance over time as the tire wears. And that combined with the possibility the mechanic didn't do a great balancing job in the 1st place could easily lead to a tire well out of balance in short order. Before you take it in for rebalancing tho, try removing the balancing weights and see if that affects it at all.
