rt4wd question

I finally started driving my rt4wd wagon this past week and noticed something strange. When i am driving on the interstate the rear seems to get power off and on in a 10-12 second intervals....10 seconds front wheel drive and then 10 seconds rear wheel drive...etc. Only when i reach about 55 mph did i notice this. if i say go 52 mph...no funky stuff. aslso if i go about 65 it stops. is that crazy or am i??
thanks for any info

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Dennis
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