Unable to remove differential filler plug
 

I have a 2001 chevy silverado 1/2 ton RWD. On the last oil change the boys in the pit couldn`t unscrew the differential filler to check the fluid level. Up to this last oil change it would unscrew ok . Anyone have ideas what caused it to seize between oil changes or how to loosen it . I tried a little of liquid plumber but no joy. Do I need to use a breaker bar instead of my 12 inch rachet. What damage if any if use a electric heat gun to loosen the filler plug.
No bad sounds from the rear end just want to loosen the filler plug for the next oil change in a couple of months..

Thanks
Bill

try pb blaster. if you use the heat gun on low heat it might work. maybe try having someone loosen it while another person uses a hammer against the diff. the vibrations might loosen the plug.