dim headlights, seem even worse at low idle speed. already changed alternator for other reason, any thoughts? thanks!

Battery to engine block ground.
Engine block to firewall ground.

AC Delco replacements at Rockauto about 50 bucks.

Put a meter on the 12 volt big lug on the back of the alternator first. Then check voltage directly at the battery Average is 13.7-14.2. Could be a chassis ground also. With the headlights on, Check actual voltage at the headlights themselves and the ground.If the voltage is ok at the headlights
String a separate wire from the ground side of the head light plug to the negative side of the battery. See if they get brighter.

If they do increase with the little ground wire,
there is your problem. I have had a lot in the past years even with the semi's. Poor ground connections. Salt corrosion/bad connections..

Two fixes. First, dedicated ground for each light, making sure it's a GOOD ground. This includes checking/fixing the current grounds as already stated.

Second, install a high amp relay that feeds the lights directly from the battery. Use the factory wiring to control the relay. Larger wiring and direct feed means near zero voltage drop to lights. Battery connections must be clean, tight, and as already mentioned, charging voltage AT THE BATTERY MUST be good. It should be the same as the output terminal on the alternator......

Corrosion attracts all wiring eventually, so check all your connectors, junctions, etc....

Check down by ac compressor these trucks have a braided strap for ground that turns green and causes all sorts of problems I’ve gotten to the point just make a new cable and add it to all these trucks engine to frame

First check is a voltage check at battery with a DVOM then at the alternator main terminal. Then check the terminal at the headlamp with the meter grounded at the battery, then ground the meter at the headlamps plug and see if there is a difference in readings.

I have the same problem with my truck, but only after I drive my wife's new car with really bright lights.