the exact FET I was using was a STP65NF06, which is also a N-channel MOSFET and has a body diode the same way a IRFZ44 does. I thought the same thing until I burnt myself, did some google-fu and ended up here:
the right diagram essentially has the body diode of the FET accounted for and is listed why it doesn't quite work(unless it's a zener). the STP unit's datasheet says the body diode functions as an avalanche, but I either did something very wrong or it isn't sufficient. the solenoid was also very non-linear in response, which is the opposite of what I expected. added in a 1N4007(as shown in the left diagram) I had from my diode selection and suddenly the solenoid operates with almost complete linearity from ~20% to 80% duty cycles and no more scorched fingers. without the diode, the solenoid was buzzing like crazy when commanding a duty cycle between the ranges of fully closed and fully open(which was something like 50 to 70%).
something to test on the bench, if you really wanted to, I was much happier finding this out before calling a project finished.