Codes 21 and 28

Code 21 and 28, these are oxygen sensor signal, They are not the cause of the problem with the shaking going on but the result of the shaking, In other word you have a lean misfire, start of with a inspection, do a visual, any vacuum lines disconnected that will cause low RPM, that you are facing, RPM at idle should be 750 to 800 RPM. Look at the MAF (where the airfilter is) there is a intake air duct these have tendency to crack or rip, so look for that If its torn or rip that the problem and new air duct is the solution, If all is fine you will need to go with a tune up, the reason is sparkplug wire can cause misfire that leads to the shaking, a distributor cap and rotor, especially the coil, that needs to be inspected it has a tendency to crack across the exposy giving low power

Great answer but I would start with a compression test due to the age of the vehicle

Roy