1993, 2WD, Manual
There is already a decent amount of vibration when the truck is at idle. However, when I turn the A/C on there is much more vibration.

Thoughts?

 

You have a misfire at low RPMs, assuming it runs smooth when driving

All 4cyl engines will vibrate at low RPM, because they only fire every 180deg of crank rotation, which is why their idle is usually set at 800rpm, below that it can get very shaky
So any misfire will be felt easily, add a load to that, like AC or power steering(turn steering wheel at idle) and you feel it even more

You can check for broken motor mount first
Open hood
Set Parking brake, or block 1 tire on both sides
Start engine
Put in 1st gear
Foot on the brake
Slowly let up clutch pedal and you will see engine start to lift up on one side, should only lift a bit, then quickly push in clutch pedal and listen for a "clunk", it you hear a clunk the motor mount is broken

Repeat test but use Reverse gear so engine lifts up on the other side


Misfire
You have dual spark plugs for better power, both spark plugs fire at the same time, no alternating or there would be no point of having dual spark plugs
Test you should do once a year
Each coil pack has a 3 wire connector
Unplug the 3 wire connector on the coil pack for drivers side, so no spark on those 4 spark plugs
Start engine
The 2.3l Lima engine ran fine on 4 spark plugs from 1974 thru 1988, so should still run fine, lol
If you have a steady misfire then a spark plug or wire is bad on passenger side, maybe coil pack but less likely

Shut off engine
Plug drivers side coil pack back in and unplug passenger side coil pack
Start engine
If it doesn't start then 1993 still has the "spark inhibitor" circuit, turns off drivers side coil pack while cranking engine for hotter spark on passenger side
So plug back in passenger side coil pack and start engine
Use insulated pliers and unplug passenger side coil pack while engine is idling(both coils are working with engine above 400rpms)
Same test as above, if there is now a steady misfire then spark plug or wire on passenger side is bad

Benefit of dual spark plugs is more power, drawback is you can never tell when 1 spark plug is not working, lol
Which is why its a good idea to test it once a year

Compression test
Usually you would do this first, because its a black and white test no grey areas like spark and fuel issue
Cold engine
Remove the 4 spark plugs on passenger side
Test each cylinder and write down results
All should be within 10psi of each other

A cylinder with lower compression can still fire at higher RPMs, but it can't add the same power as higher compression cylinders, so vibration