The output from a guitar pickup will be higher - it can be many, many times higher - than a microphone's. In the extreme, say comparing a passive ribbon to a high-output humbucker, the difference might be 60-80 dB (guessing those numbers off the top o' my head). An SM58 has a much hotter signal than a ribbon, but it's still a tenth of a typical guitar pickup's output.
As proposed above, one solution is to add an inexpensive microphone preamp to your setup. This can have added benefits, as some preamps also offer EQ and compression, making the process easier when you are both vocalist and engineer.