SOLVED!!! Step by step I backed through my math, setting variables as simple numbers, until I came across the TRUE trouble code. 'enctickL' becomes 0 when the robot isn't moving, and i was doing 1/0 which becomes NaN! I didn't think anything of it at first because it was reporting regular numbers. This was then filtering down through the math and eventually showing itself at 'heading = heading + angleinc'. I added two if statements to change them from 0 to 999999 to my main code and it worked! Thank you guys so much for the help! I'm feeling much better about bringing my robot in to class today!
enctickL = pulseIn(encpinL, HIGH, 40000);
enctickR = pulseIn(encpinR, HIGH, 40000);
temp1 = 1.0 / (float)enctickL;//temp1 = ticks per millisecond
temp2 = 1.0 / (float)enctickR;