Transmission

Okay, so it seems like it cannot make its mind up whether it wants to be in drive or over drive when I try to hold it at a steady speed. It makes a sound that sounds like the rpm's are going up and down. And makes bad vibration from it. While trying to hold at steady speed like it is in over drive when it did not need to be or either not in it when it needs to be. But I can lay in to the fuel and start gaining speed and smooths out but they are not sure it is the transmission. But also it is not slipping. I can pull out from a complete stop and as long as putting fuel to her gaining speed it does fine and when coasting not given it fuel. I feel like it is a little sluggish sometimes like something hold it back. But not slipping. I have had truck for a year it has done this the whole time I have had it.