Why does a 1993 Infinity J30 shake after replacing all the plugs?

J30's don't have a distributor. Each cylinder has its own ignition coil controlled from the engine computer.

It sounds like it is misfiring as one of the cylinders is now no longer reaching ignition.

I'd say double check the connections. Push on each plug to make sure they are completely engaged. I am talking about the power source to the coil.

Hopefully, you didn't overtorque the plugs and strip the cylinder head sparkplug sockets. Only about 15-18 ft./lbs. are needed.

It is possible that the wires from the distributor were not replaced to their corresponding spark plugs. Did you use the correct replacement plugs? The J30 requires platinum-tipped spark plugs.