Why will this vehicle not start?

Alright. Vehicle is a 93 Ford Explorer. Radio, lights, etc. All working fine when key is to ON. When the car is to start, all we hear is a click. We've pushed to start and that's worked half the time. Cleaned up the battery terminals.

Battery gets 12.6V standing and 11V when trying to start. Had starter checked twice, worked. Replaced fender-mounted solenoid. Tried bridging the solenoid, same click. Got 11.2V on the starter solenoid terminal ('always hot') when not starting.

Headlights don't noticeably dim. Couldn't jump-start, either.
Thanks

Added (1). That could be. I wonder why we still get 11.2V, though. When starting, the current between the "always hot" and only-cranking terminals zeroes out. (I'm thinking that the current was flowing properly between those two. Either that, or starting actually turned it off.) Didn't test the third.

Maybe you have a problem with the battery cables themselves. I had a Jeep Cherokee that experienced a similar problem. I had to replace both battery cables.

You're on the right track doing the voltage drop tests but you need to go to the actual motor terminal when someone is twisting the key, also when checking the other voltages you mentioned you need someone to try and start to track down the bad spot. I suspect you have a bad ground or connection somewhere but there's one huge problem, nobody can really test your starter without bolting it into another truck. A bench test proves very little as the motor is not loaded.
If you have something close to 10V at the actual starter motor terminal when someone is cranking then you have a bad starter.