Subaru trans get stuck in gear

2005 Subaru 2.5i with five speed. 150k mileage. 
Twice in the last few months my shifter has gotten stuck after parking the car on level ground. I have owned this car for 18 yrs and almost always put in in second when I park and set handbrake. When it gets stuck it feels like it is almost between 2nd and 3rd. I have to rock the car back and forth to free the shifter. 

What are likely culprits? Shifter linkage, shifter bushings, shifter forks, synchro, other? Once freed the car shifts just fine. 
 

I am due for a clutch so I would like to have this addressed at the same time by my mechanic. Just wondering what to expect. I would rather not have to have the transmission cracked open. 

It's unfortunately unlikely to be anything external to the trans as the Subaru shifter is a single rod that rotates and is plunged in and out.  

Internally there are 3 rods with aluminum shift forks attached theat move the gears. 2nd and 3rd are on different shift rods.  

There are a series of balls and pins between the shift rods that interact with V shaped notches in the sides of the shift rods.  When any shift rod is moved out of the neutral position it pushes three pins into the other shift rods notches preventing them from being simultaneously engaged.  It's definitely possible that these pins or notches could wear.  

But in my opinion it's more likely the aluminum shift fork is worn or bent.  The shift fork runs on the shift ring with just a plain bearing.  It's possible for the pads to wear.  If the pads were worn in the neutral position you could have 2nd still slightly engaged when you try to engage another gear.  The syncros are wide enough that I would think this would be unlikely but I guess anything is possible.  The pads on the shift forks seem to be about the thickness of the syncros so maybe.  

The good news is Subaru 5MTs are pretty cheap so you probably could just locate a "good one" and swap it.  There are few revisions so it shouldn't be difficult to find one that is the same Ratio.  

Engine on or off?  What if you put it in first?  Regardless I would change the fluid and have a close look at the old stuff for metal.

In reply to nocones :

Wow great info. Thanks

re porshenut- engine off, can't really start it stuck in gear. I do wonder if it is low on gear oil. 
 

we used to have a Subaru service center in my city, but COVID killed it. 
Bringing it to my mechanic tomorrow. 

If it is stuck in gear with the engine off it seems like a fluid check might be too late.  Something important is binding.  If you are lucky the linkage is blocked or bent, if not it is internal.  Start looking for a junkyard trans.  Compatability is pretty good across the models and years.

I bought an outback once, Arizona car that had no rust anywhere.  Heat shield bolts looked new.  He towed a uhaul trailer to PA, which must have stressed the trans so it developed a fifth gear popout issue.  Drove it anyway, just held the shifter when in fifth.  When not in fifth I beat the crap out of the tranny.  Rarely used the clutch up or down shifting.  And never had an issue.  So they are pretty tough and finding a working one from a rusted out car should not be tough.