The wooden back door to our house is badly warped due to Oregon rain
and constantly shifting temperature changes and once it's open it's
hard to close. But once it's closed, it's bloody difficult to open.
Does anyone have any suggestions on how to fix this?

The cheapie route would be to refit the door, shaving/sawing bits off sides
until it fits with some play, as long as the weatherstripping stays put. I
have given up and replaced two such doors with prehung metal doors, replete
with jambs, shaped/sculpted to look like regular wood. You can get them with
or without double glazed windows. Lowes and Home Depot have them at
reasonable prices. They don't rot, and seem rust resistant.