Transmission Noise

[Avs_Fan]

I am experiencing a noise that I believe is coming from the transmission of my 81 CJ5. I have had a friend of mine (mechanic) listen to it, he said it is coming from the transmission. Especially since it is not as bad in 4th gear. He suggested changing the oil (I was not sure what was in it). I did this tonight, but the noise is still there. Am I looking at rebuilding the transmission? I was planning on pulling the engine this winter and going through it, guess I could do the transmission at the same time? Does anyone have any other suggestions? What can I expect to find, how much can I plan on spending, would it be better to find a rebuilt one or used one? Lots of questions at this point, thank you in advance for any suggestions. I understand it is hard to diagnose a noise when you can't hear it (I have a hard time explaining it).