Worn synchro. Good news is 5th gear is easy to replace, but while you're in there you mareswell do the whole lot.
Good news is that if it's the first time you're transmissions been worked on, as long as you just take them of once and put them back on, everything will likely just slide right on and of the shafts. Otherwise you would ideally use blocks and a press to seperate components. That or 3 jaw puller. The helms reccomends not using pullers, but understandably not everyone can afford use of a press. Just know that if you somehow do nick the working surface of a gear or something, it's done.
Man trannies arent complicated, just make sure you have a shop manual and a place to keep track of all the bits. Double check everything as you put it on and you're fine. It's really really not hard.