I removed every lock motor on my (3) Cherokees, opened them up, and lubed them with Mobil1 oil. The front ones work fine now, and the rear ones are intermittent, working best while the engine is running.
I suspect that the very small wires used these days coupled with the number of plug connectors in the circuits delivers insufficient current to power the lock motors. I plan to install larger gauge hot wires to the doors and use relays to feed the lock motors. I'll try this on one door (the right rear, because it's farthest from the source) and if that works, then the Left rear.
I strongly recommend lubing the lock motors first. If you decide to try this, make sure that the "hot" wire is fused... you don't want a fire!!