Engine shaking when idle

Possibly a vaccuum leak in a small line.. Would cause the engine to idle rough.. Also a bad sensor valve.

I really need to know the mileage.....?..also the question isn't very clear ...Is the whole truck shaking roughly or is the engine shaking roughly or is it both the engine and truck shaking..?..Since I am not sure I will answer what I can....Ok......If the truck is shaking while sitting still at idle the engine/trans mounts could be worn and or loose mounting hardware...If the engine is running roughly at idle causing all to shake that bad it could be a number of things causing it as usual...Quite often a faulty plug wire/and or bad plug will make it have a misfire because it will be running on only 5 cylinders..If that's the case it could be more than one bad...A vacuum leak will cause it to perform in the same manner...however all cylinders will be firing..Normally a vacuum leak is easy to pinpoint..however it could be a faulty IAC valve..(IDLE AIR CONTROL)..The idle air control is designed to hold a high idle at start up then slowly bring idle down to proper Rpm.....A faulty TPS (THROTTLE POSITION SENSOR) will also cause a rough running problem..And let's not forget the O2 sensor..(OXYGEN SENSOR)..The O2 sensor controls air/fuel mixture...and yes an exhaust manifold gasket will make it run rough but not too bad...It has a rough running sound more so than causing it to horribly shake..It will shake but not to bad..
it is located at the exhaust side of engine..The exhaust manifold is mounted to the engine exhaust port..the exhaust pipe is mounted to the exhaust manifold and the muffler/cat converter is mounted to the exhaust pipe...You can check the plug wires quickly..With engine running carefully remove plug wires 1 at a time..If the sound of engine gets worse the wire you removed is good..after verifying it is good or bad plug wire back on then pull the next wire..and so on..If you pull a wire off and there is no change in engine operation that is the bad wire..If that is the case change all plugs and wires...One at a time..(It is a good idea to make sure all wires come off easily before starting engine..old wires will come apart easily..grasp wire tightly at boot and twist left to right while pulling..)as far as the other components I spoke as well as many others,the best,easy way is to have it hooked to a scanner and they can check/test all components for proper operation...Now..If it runs good but is shaking the engine/trans mounts are worn as I stated earlier..any loose engine component will cause an engine shake also but you should notice it at a glance...engine mount bolts/nuts being loose can be tightened without changing mounts if caught in time..Trans could be coming loose.also worn clutch will cause vibration...Send more info and I can guide you to remove and replace any faulty components you may find to save you money.........Brooks/Metalpoet.....

check vaccuum pipes may be at fault and need replacing.

run fault codes and look for position sensors --MAP/MAF/VAF sensor problems and fuel /injector faults.

could be a vacuum leak at a hose or gasket. with the engine running spray a little carb spray at the intake manifold area. if the idle picks up you are close to the vacuum leak. dont spray on the exhaust manifold area, be careful

Spark Plugs have to be changed every 3 years
Front Oxygen Sensors changed every 100,000 miles
Can't have any Vacuum Leaks -- anywhere on the vehicle
Need to have it road tested with a Professional Scan Tool
& look at all data & get caught up on Preventative Maintenance
Very few people change motor oil 3 or 4 times a year, if at all,
and the Variable Cam Actuators fail, as a result,that & few other
things can cause rough running engine & resulting idle issues
It's well known that Ford COP Ignition Coils fail sometime in
every year of ownership

I would check for a broken vaccuum hose . A vaccuum leak will raise the idle considerablly.

I would check all wires and hoses that has been disconncted during performed repair.

sounds more like a air control /sensor but its difficult to diagnose from spain without seeing the car ,sorry for being able to give a quick fix

your catalytic converter could be clogged.