Ford explorer 2000 model auto 4.0l slipping/ clunking noise coming from front end?

Does anyone know what this could be i have a 2000 model ford explorer 4.0l xlt 4x4 auto when going up hills there a sort of squealing /clunking noise coming from the front doesn t do it all the time just manily going up hills or when i use a heavy foot its not the auto slipping the revs don t change or doesnt free rev when it does it. My thoughts are that the front drive shaft is worn but could anyone help me out it also could be a belt pulley cause it sounds similar.

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