So I've been reading up as much as I can about this and it seems a few things can cause 5th gear to pop out.
I'm having trouble finding a case that is exactly like mine though and don't want to just start replacing things.
2003 6 s 3.0 5 speed, I've owned it 2 weeks.

5th pops into nuetral only when given a lot of gas, it will coast and light throttle in 5th just fine. Every other gear works just fine no matter driving style.
If you give it enough gas (especially at higher rpm) It pops out even if I'm holding is as tight as I can in 5th, that is not to say the shifter is being pushed into neutral. I'm a larger guy and it pushes back on my arm until it feels like it's gonna break the shifter, even if I don't let it budge it will eventually go into nuetral and if I keep a hold of the shifter and let off the gas it pops right back into gear.

I'm at a loss, just looking for some help. At first I thought for sure motor mounts letting to much movement but then when I held it in gear it popped out and back In with no clutch at high rpm. Makes me think fork or gear teeth.any advice is appreciated.

Side note. Did this once last week going up a steep hill, put it back in 5th no problems. Yesterday i couldn't drive in 5th with anything over quarter throttle.

 

Going by my experience with early 90's nissans with their 5th gear pop out, its due to worn components in the gearbox. You have 2 options, rebuild or 2nd hand replacement.

 

I had a similar experience with a early 1990s V6 equipped Mazda MX6; the issue turned out to be a worn shift linkage. As for your issue, it is pretty hard to diagnose over the internet; best you take it to a shop who can look at the issue first hand.