It doesn't take much differential before you can't open the doors.
Most doors are plug type so the pressure keeps them shut.
Normally there is a link to the squat switch which opens the dump valve and there are also moisture traps in the lower hull which open at low differentials to get rid of the condersate thats formed on the inside hull.
If that all doesn't work there is a emergency dump valve which can be opened.
But to be honest once you turn the engines off the door seals would deflate and the air would equalise. But most aircraft are tea bags with holes in them and as soon as you turn the flows off they equalise in a matter of seconds.