Long story short, did the whole lost power at high RPM thing. Replaced Battery (6 years old), serpantine belt, tension pully (it looked like it was about to fall apart) and alternator. Gotta appreciate a good autozone, tech there helped me with all of it.

The RPM's on my truck have dropped now. Is this normal? Was that old alternator (probably 16 years old.... my grandfather never did any maintenance on this beautiful truck, lol) forcing my engine to work harder? I would say drop is significant. Down from 1500 to 800 at idle and from 2900 to 2000 at 65/70 mph. Just want to make sure this isn't due to some other oddity or something I didn't realize.

Grabbing popcorn and sitting back, hoping for insight. Truck is a 99 ranger xlt supercab 4wd with the 4.0. She's Purtty.

 

With automatic warm idle should be about 750-800rpm
Manual trans 650rpm

And in Overdrive(.75 ratio) 2,000-2,100 RPM at 65 would be about right.
But this is also dependent on rear axle ratio and tire size

I don't see any alternator connection, outside of tach showing incorrect data because of electrical issue, bad alternator can make for a "noisy" voltage and tach is "noise" sensitive.