Car is shaking when idling - 1992 Volvo 940

Look for a vacuum leak.

Hello there , if if idol is the same rpm as normal , Try looking at your engine mounts and transmission mounts , if one of them are broken , this can give bad vibration at idol .. Regards Graham ..

A shake usually means a cylinder miss.
Is the check engine light on ?

What about new spark plugs and wires?

if it goes when you put your foot down a little bit, then it sounds to me like idle speed control valve

your idling must very low thats why your car shakes..check the rpm gauge and refer to your vehicles manual on proper rpm when idle..bring to the mechanic to adjust the idling rpm

old engine mounts can cause that problem, even if they look ok sometimes

Shakes how? While stopping? Or solely while idle, Engine light on?

If if shakes while stopping you need to check you're wheel hubs and or Suspension ,

Shakes at a light, You may have a bad fuel Injector, Check you're psi in you're fuel pump, A TPS would cause low idle or sometimes high idle, With low idle and a bad motor mount it would shake,

I know theres a few things I'm missing but,.. I have a major Headache atm so I'll get back to you with some more, when i can think better. :)

Suggestion only! If you had no shake before the tranny work,then retracing the steps backwards is a good way to find the new problem.
Go back to the tranny shop and complain #2 go to another tranny shop and get a professional to look at their work.

This can be your Idle control valve, when you turn the air on it puts the engine under load and your computer will increase the idle automatically to prevent the engine from cutting out.
When you are standing still and the car is shaking, by touching the accelerator very softly does the shaking stop? If so check the idle control valve.
I hope this will help.
Regards
Sam