I have a 2000 ranger xlt 3.0L 4x4 4 speed auto, about 170,000 miles

recently when climbing hills going 70-75 using cruise control on highway, it will sometimes downshift all the way down to 2nd putting the RPM's up at 4500-5000. i've only been getting around 15 mpg since dealing with this issue.

head gasket was replaced less than a year ago, and i've never had this problem until about a month ago after buying the truck last march.

anybody know any reasons why this could be happening, or is this pretty normal?? any help or insight i could get would be appreciated

 

my old 99 3.0 4x4 w/ 4.10 gears would do this ALL the time. I never used the cruise control because i'd get worse mpg when it decided to floor itself up hills lol. I owned the truck from 50xxx miles to 114k