My Automatic Gear box is not switching gears. Actually it is not switching from 3rd to 4th gear I think. What happens is that the RPM's go really high after 3rd when trying to switch to 4th and the engine suffers a lot. After a few seconds, it switches but don't know what can be happening here. Actually this is something that happened 2 months ago, and now after using the 4WD just a little bit it started happening again. 4WD was also not working right. it was making a lot of noise and sometimes when switching to 4WD, the gear was not even engaged. Some days. it works ok. Don't know why. The transmission fluid level is ok.
What could it be?

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2006 Jeep Cherokee Liberty 2.8 CRD