How can i select 4 wheel drive on my 06 jeep grand cherokee it has an auto gearbox thanks

You need to be more specific. To start, which 4 wheel drive system your jeep has? (Select Trac, Quadra Trac II or Quadra Drive)
Then: does it never work, or only in 4-Lo?

yes, but make sure to find out which wheels are getting the power

Your jeep should have full time four wheel drive. To place the transfer case in low range look down to the console towards the rear and you will see a "T" shaped handle. To place it into low range you should stop place the vehicle into park and lift the T handle and in the dash a 4X4 indicator lamp will light up. The traction button on the dash would be used in 4 wheel high mode when you start to experience wheel slippage upon take off from a stopped position. This will give you a little more control. You only need to use the low range for very slow speeds in mud or snow. Otherwise just leave it in high range because it is always using 4 wheel drive. This is a very reliable system which Jeep has refined over the years. Your Grand Cherokee is not designed fro hard core off road use. Hard core off road use will destroy the undercarriage. The Grand Cherokee is more of a luxury soccer mom 4X4 with good road manners, a comfortable ride, and good reliability.

Hi

Full time 4 wheel drive should be used on rugged and difficult to drive on terrain, this could include adverse weather such as deep snow, mudslides, sand dunes etc.

Part time would be used on terrain that is irregular where some parts of a track may be road and other parts rocky, mountainous regions combined with any of the above weather conditions. The 4 wheel drive should not be used for normal driving as it will only over time damage the gearbox, the 4 wheel system is not designed for speed but purely for grip and stability.

Hope this helps
Regards
Dan

near the gearstick you have another one, on which it writes 2, 4 , and low, if you put it in 2 you have only the back wheels powered, if 4 you are on 4wd and on low you have a power gear, that gives you more power on the wheels, by changing the gearbox ratio

If your in 4 low be sure to shift into nuetral first. Otherwise try backing up while shifting to 2 wheel drive

Throttle position sensor

No, not with out a lot of work. Electronics are most likely different on the 2 also.

I HAVE MY SOLUTION TO MY JEEP PROBLEM THANKS FOR ASKING AND TRING

You do it with the engine on and put it in neutral. To place it on 4WLO or 4WHI. It all depends if you have a auto 4 wheel drive these type engage the 4 wheel drive automatically when the wheels spin to about 16 MPH. Then there are the ones you press a button to engage it to 4LO , 4 HI or 2WDrive. The last is you have a lever that you manually place in whatever position you want. 4LO, 4 HI and 2Wdrive.