1998 5.9 Durango is not correctly shifting from 4WD to 2WD.

The manual shift lever is all the way into 2WD and the light in the dashboard indacates that the operation mode is 2WD; however the front axles are not

disengaged.

After about 5 mins of driving the vehicle has a loud "clunk" and then it is truly in 2WD.

Please post any ideas, suggestions, indications of component failure?

Thanks in advance!