vibrations

Symptoms indicates a drive train problem the the item most likely to cause such vibrations would be the propeller shaft center support bearing.

Have it checked for damage to the bearing support and excessive side movement. Have the shaft U-joints checked as well.

It has no center support bearing,
can anything else cause it.

Whee is the vibration most felt?

Were the engine and transmission mounts checked?

yes they were checked and the tranny mount was changed

Noise and vibrations are never easy to diagnose what more without the physical vehicle. You need to try to locate the source of the vibrations and go on from there.

Does the vibrations occurs when transmission is shifted to neutral?
Is it related to engine or road speed?

I would say its road speed,when the van moves off and tranny changes to 45mph,the vibration starts,when you press the gas pedal the van picks up and after speed is reached and starts cruising it starts the vibration again,

That can mean engine speed, road speed or drive train torsional load related and with limited information, I am sorry I don't think I can be of much assistance.

what I observed is that when the van is cold it runs fine after it gets hot the vibration starts.only after 45mph and over.

Is the Check Engine Light indicating?

The additional information indicates it could be from the engine, maybe a misfiring.

the check engine light does not come on,I took it to auto zone and they put the scanner on it read the #2 cylinder I change the spark plug and the coil and it still does the same.should i change the injector.

Try swapping the injector with another cylinder and see if problem migrates.

Have a compression test done to see if it is within specs.

the spark plug tip color is white does it means something

That is an indication of a lean running condition and could be the injector is not firing correctly. It could be an injector, its wiring, engine computer problem or cylinder compression fault.

I changed the injector,and its the same,only happens after reaching the operating tepm.and crosses 45mph.
can a knock sensor cause this

A faulty knock sensor would usually throw a code and can cause the problem. Is the Check Engine Light indicating and have you tried retrieving for trouble codes.

Have you checked the clinder compression? It coudl also be due to a faulty EGR system ( if equipped).

the check engine light works but does not come on to indicate that there is a problem

Get the codes read again to see if the problem is still cylinder # 2?

You have not performed a compression test yet. It could be they key to an answer.

i also observe that when driving 50mph or so and shift in N and press the gas the engine runs clean and smooth but when shift back to D the hesitation starts back.if that can help any

In N, there is no engine load so any minor misfiring would not be noticeable, especially at higher rpms.