Back in 2009 ,I bought a new 2009 Colorado 4x4 automatic 5 cyc .Im the only driver at 147,000  miles and going strong. 

 I live in SC so the truck is  in Excellent condition and the 4x4 has very few miles  on it. "no snow here"

its been a good truck, It is mechanically  perfect, until a few days ago at 45 or 55mph  steady  speed the transmission is shifting from what  "I guess is  overdrive " or 4th gear to 3rd , I see the rpm  jump also.

I plan on keeping the truck for a long time. Any Idea what it could be?

or should I just replace the transmission?

checking around I can get a rebuilt transmission for around $1800 then $1200 to put it in , "a lot cheaper then a new truck"

Thanks for your time.  Dave in Charleston sc