This could be a lifter as you are suspecting. However, the 3.8L was known to have issues with the A/C compressor starting to lock up. Remove the belt and run the engine for a few minutes to see if it does it. If it does it with the belt off then we know it is inside the engine. If it does not then that make a little more sense as to why it does it with the A/C on.
If this dose not make a difference then we need to monitor the fuel trims with a scan tool while it is running rough. Next we need to check for a floating valve. Intake valves need to be checked with a vacuum gauge. Exhaust valves can be checked by holding a dollar bill over the tailpipe. If it feels like it is being sucked in the you have an exhaust valve hanging open.
Let me know with all this info and we can go from there. Thanks
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Sunday, August 9th, 2020 AT 6:37 PM