Hi Salsaman,
If you are using softwareSerial, the two serial communications should be unrelated and you should have no problems.
I cannot find the LCD you are using on Sparkfun, so I will give you my best guess:
Your function to print to the LCD is interrupted by the ShiftPWM library and the LCD controller can not handle a long pause between the received characters. Could you try using this:
cli(); //disable interrupts
/* print to lcd */
sei(); //enable interrupts
It's just a guess, because I cannot test it.