95 dodge ram 1500 4x4 5.2 downshifting problem?

i have a 1995 dodge ram 1500 4x4 with a 5.2 v8 the truck has a 3 inch body lift with 33 inch tires when i drive on the interstate after driving a while it seem as if the transmission down shift and and the rpm goes up and wont go back into high gear the transmission is an AT with overdrive i have no clue what could be causing this to happen or if it is even a trans problem if anybody knows please help thank you