I have one receptacle left to replace and it was back-wired. The two black wires had a release slot, so I could remove the wires. The two white wires do not have slots. Any suggestions?
The wire tends to be abused most (bent and re-bent, often sharply) at the back of the device, weakening the conductor. Wires with nicks have increased resistance because much current flow is at the surface of the wire (to oversimplify how electricity actually works). Also, bare copper tends to oxidize over time, reducing conductivity when reinstalled. It's best to start fresh when possible.
But... when I have to remove a receptacle with wires in the backstabs I pull and spin. The locking tab inside will essentially cut spiral threads and the wire will eventually come out. If you can't spin completely, twist as far in one direction as possible, then reverse. Repeat until it lets go.