Alternator charging system help

2008 5.7l. Truck died on me while at idle on highway with accessories on. Dash gauge was dipping toward the 9 and bearing was making noise so I pulled the alternator and installed a denso reman. New alternator is idling very low and so I had Oreilly test it. Told me voltage regulator on new alternator is bad.
Took the old to a shop. They bench tested it and told me the old alternator (also a reman denso) was good.
So… either I have a charging system issue or both alternators should be good and are actually bad.
Tested the newly installed alternator at the positive post and alternator nut and got no voltage drop. Also acceptable resistance across cables and battery. installed new battery. Installed new 7.5amp fuse under dash. Installed new battery terminals. Same issue.
To be clear, here are my voltages:z a
Truck running at idle: 13.8-14.1v depending on accessory draw. Alternator is charging the battery unless I put the truck into gear. In gear the truck runs at 12.37ish unless I rev it. RPM around 400.
anyone got any thoughts on this?

im at a loss.

 

Update: fixed it. Dirty throttle body was causing too low an idle. phew!!

 

I would take a look at the battery... If the battery is starting to go, it will die in gear as well, just ask me how I know... and how mine would often drop me off in the best places, I mean pitch black couldn't see my hand in front of my face, because there can't be a better place for the battery to die /s

 

Low idle is normal when replacing the alternator, as the ECU has to relearn proper idle speed.
There is a quick procedure you can do for it to happen right away (you can google it).
Otherwise it will go back to normal after a few days of driving.