Wow. This brought back memories.

Over 40 years ago that was the problem with my first car, a '65 Mustang. I just loosened the terminal on the starter and scratched around a little on the mating surfaces of the cable and the starter. There was no visible corrosion, but the connection was somehow not good. Solved the problem.

Where are you measuring the 12.6 volts when not cranking? If you're getting the reading from the battery posts you might want to check downstream, at the starter perhaps. A lower reading at the starter would confirm a connection problem.

On the other hand the problem sounds like insufficient amps making it to the starter and not low voltage, which again indicates a bad connection (or two, or more). And just like with my Mustang, the connection can look fine and still be wonky.

 

Yep when I got my rusty but trusty 97 XD it was weak starting and I had ECU voltage faults, and I didn't even have decent voltage at the OBD2 Port. I cleaned all the battery terminals, no change, cleaned the ground to the frame (helped), cleaned both small and larger terminals on the starter (fixed slow starting), and eventually tracked the ECU issues/OBD2 low voltage down to a completely rusted ECU PCB..... Slapped in another non rusted ECU and it's ran like a champ ever since. Starter terminals looked ok, but once cleaned up it was night and day. Original starter is still in place as well.

 

I’ve read about that same grounding problem at the firewall from other Disco 2 owners.
As a safety precaution for me I wire brushed that grounding area at the firewall, the rear grounding point on the intake manifold and wire brushed the webbed grounding strap. I also added an 8” length of 10AWG cable as an additional ground to the firewall from the coil pack’s top mount bolt.
You can’t be too careful...

 

Starter replaced - issue appears to be resolved. Thanks to everyone who weighed in - really appreciate it!

 

What if it does start without the donor car running? What needs fixed? I have to jump her every day!

 

Here are your troubleshooting steps:
1. Head to a local parts store and have them check your battery and alternator. Perhaps there's a bad cell in your battery and it doesn't have enough CCA to turn the engine. Could also be an issue if you're traveling in really short trips that don't provide enough time for the battery to charge back up to a level that can restart the truck.
2. If the battery and alternator are OK, check your grounds at the block and body as well as the battery terminal connections - make sure they're clean and tight.
3. If the grounds and terminal contacts are good and clean, check your starter contacts - make sure the battery is disconnected for this.
4. If issue is still present after the steps above, replace starter.

I ended up needing to replace the starter.