Motorcycle Questions? Ask a Mechanic.
Hi and thank you for your question.
What happens or what do you feel when you attempt to shift into 5th? Is it like another neutral? Does it not want to come out of 4th? Does it grind or make unusual noises when you try to shift into 5th?
Thank you for the additional information. Since you didn't specify, this answer applies to the TM, not the ES. If you have the ES, let me know.
I suspect what has happened is there is a problem with the pins on the end of the shift drum. This would be the most common failure associated with your symptom. It is not a slam dunk guarantee, but it is the odds on favorite. You will need to take a look at it to be sure. In the extremely unlikely event it isn't in the mechanism, and is an internal transmission problem, this would have to come apart anyhow. However, I am almost certain this is a shifter linkage issue, and not an internal gear problem.
Here is a diagram of the shift drum and shifter mechanism. The pins I am referring to is reference #39. You are going to have to take the clutch cover and clutch off to get to the shifter mechanism, but at least you should not have to split the cases.
If you have any more questions, feel free to ask.
Have you had a chance to look at this yet, or is it something you don't feel comfortable tackling yourself?