Any help onoverdrive issue.

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Ok heres the skinny. I baught the truck used in december with 156k miles on it. I then drove it to vegas and back with no problems what so ever. i even hauledback about 6k pounds of trailer and hay. The trip took place around new years. Anyway the issue is about late january i noticed it wasnt staying in overdrive. I could set the cruise and it would be fine on the flats but the littlist hill would cause it to downshift. One timei noticed that od light was blinking and after reading alot of the forums i changed the vss on the dif. Today i also got an oil change and tranny service filter chage not a flush. Herd to many horror storied about people getting the flush done and boom tranny explodes. The truck now has 160k miles on it and im at a loss on what to do next. Also no codes are spitting out and yes i do have a scanner. I also changed the fuel filter today I wanted to get a fresh start on all the fluid changes since i baught the truck used. Is this normal for these trucks to downshift on the smallist of inclines or is there an issue? When i first got the truck i dont remember it downshifting so easly.