The ignition key should open the door locks - unless the doorlock has been changed
do you mean the door or ignition?
Yes it should.
The rear hatch on a 1998 dodge durango won't open
It did when it was built.
Yes
No, tank has to be removed.
Unless one of the key cylinders has been replaced, one key should work both.
It's supposed to be
Driver side door must be successfully unlocked by the door key(I have a separate ignition key).
The obvious choice would be to use the key in the door.
On a 1998 Ford Explorer : Yes ( as long as none of the key cylinders has been changed )