Can I engage and disengage the 4 wheel drive while I'm driving? At what speed?

Possible for a speed sensor failing. Not sure on those trucks.

You have to put it in neutral with the engine running and the vehicle stopped and your foot on the brake. Press the button and wait a few seconds and you should hear it disengage. You have destoyed the deferential by driving it on a non slippery surface with the 4x4 engaged. Very very expensive mistake if you ever decide to fix it. Hopefully it will do ok in 2wd.

For this problem,click the link below and go through troubleshooting procedure:----4 wheel drive system will not disengage? http://technoanswers.blogspot.com/2012/01/4-wheel-drive-system-will-not-disengage.html
---------4x4 front differential will not engage the axle on ford? http://technoanswers.blogspot.in/2012/02/4x4-front-differential-will-not-engage.html
---------This will help.Thanks.

There can be a linkage adjustment problem. Also you may need to remove transfer case housing to check the spring inside. There is a chain inside. That one should be checked for its tension. If those are ok you may need your transfer case get serviced.

sounds like a problem with the transfer case

To engage the four wheel drive, while rolling, pull the lever back one notch and that will engage the transfer case. You can easily do that up to speeds as high as 35mph. Pulling the lever all the way back will put it into 4 low which is not often used and may grind if it does not engage right away. That really doesn't matter much because that gear is only used for pulling heavy loads at low speeds. (I use mine about once every two years) If you do need to use it, generally you can engage it with the engine off without problems. The neutral position is used to disengage the wheels if the vehicle needs to be towed. If it fails to disengage, often backing up about twenty feet or so will help.

The hi/lo is a function of the 4 wheel drive on the vehicle. The hi ratio would be used when negotiating sand or mud for instance and the lo for steep inclines where more power is needed. This is like having a gear with better pulling ability than first gear. You will find driving in 2nd gear lo will give you the same speed as 1st gear high.
Do not engage the four wheel drive system when driving on hard surfaces ie tarmac or concrete. Use it only on surfaces where additional traction may be needed. It is a good idea to use the 4 wheel drive system at least once a month in order to lubricate those parts of the 4 wheel drive system that would otherwise be "dormant" when in 2 wheel drive.
If you have diff lock funtion on the vehicle it would be a good to idea to engege this as well but once again only on a soft surface. The reason being that the centre diff lock tends to bind and stay engaged due to it not being properly lubricated.
If it does bind(dash indicator stays lit) find a patch of grass or sand, engage reverse and drive a couple feet. If this does not disengage the lock turn the steering fully left or right and drive in a circle. This should then disengage the difflock.

Be sure to put the truck in neutral, while engine is running, while you disengage from 4 wheel drive (low) to 4 wheel drive (high) and then finally to 2 wheel drive. It is critical to shift transmission into neutral when switching out of 4WD Low, as indicated in your owner's manual. Hopefully, this will help to FixYa problem!

What type of vehicle is this?

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You first have the car running and as long as you are going or driving under the speed of 55 mph you can put it in 4 wheel drive high. for low come to a stop put in neatural and switch to low. put in drive and go. max speed 10 mph