Reverse grinding without apparent reason

Hey guys,

Been a long time reader of the very helpful posts but this is me finally registering and posting

So my wife has a 01 Jimny. About i would say 2 years ago (not sure, but it's been months and months) she complained about 1st gear being sometimes a bit hard to shift and reverse grinding when clutch was fully depressed. We quickly avoided reverse and brought it to our local mechanic and he said the clutch pedal assembly was torn so he fixed it and stuff went away. Fast forward last summer and it did it again. Dropped to the shop again, fixed. Mechanic didn't elaborate much (didn't ask much either tbh) but mentionned the clutch would need changing in the near future. I thought i was going to handle it next summer as part of a bigger maintenance.
However back in november it started doing it again. So i investigated and decided for an oil change as it was supposed to be quick and easy. Turned out the filling plug was machined by the previous owner and so i had to remove the box in order to make some room. Long story short, ended up changing the clutch since it was on its way out. Everything worked flawlessly for 1.5 month. Wife was happy, gears were easy to use again.

Unfortunately 2 days ago, it did it again. 1st was a bit notchy and temperamental (ie would sometimes go in easy sometimes not). Usually when it wouldn't go into reverse, apart from avoiding it alltogether (lol) i advised my wife to just stop the engine, go into reverse and start it over. Better kill a battery than the box. Anyway she did just that but reverse wouldn't go in at first. After just a bit of fiddling it went in. 

I should also note that the grinding on reverse occurs when she's trying to push the stick into reverse, not when she releases the clutch. And i've used the correct grade oil when i changed it recently (don't remember which one exactly but like GL4 75W80 or 75W90 iirc...bought diff oil at the same time so i'm not too sure but i double checked before filling).

My wife mentionned 2 things :
- it seems to be doing it more often when she gets home and drives down our driveway which is a pretty steep decline then flat.
- she also mentionned she feels like it always acted up on her whenever the temperature was swinging. Past few days have been 10°C+ compared to 0° the last few weeks. It did it in november when the first cold days went by. And it first did it when moving from winter to spring.

I've did a quick search here and people mention the gearbox really needs the correct grade oil. So i'm thinking draining and filling again. I'm confident mine was the correct grade and i drained the old oil best i could but i haven't done 1 or 2 flushes so maybe there was some leftover old oil in some crevices.
Other than that, what could it be ?
It's on 120k km, my wife has had it for 15 years and always serviced it to the same shop we're going to (guy seems very honest) apart for the few big maintenance i'm doing myself but we still bring the jimny to the shop once every 1-2 years for a global checkup (in case i missed something myself):

I also don't understand why it went fine for weeks without a hiccup and suddenly acts up on us.

Thanks for any help you could provide.

EDIT : i've read that the mount under the gearbox failing is a common cause for reverse grind. Is it consistent with my explanations ? If it's the mount under the box that mates it to the cross member, i've laid eyes on it while doing the clutch and it was it decent shape. Certainly not new (probably 23yrs old lol) but still firm, no cracks, just the good amount of slight mushiness. I have torqued the screws according to the manual too.

I have. Seemed good enough. Regreased it and reclamped it. Wanted to adjust the bite but while trying the pedal it felt good enough so i left it as is

Seems more likely to be a clutch issue to me. For the gears to grind for more than a moment they must be being driven with some force, which they wouldn't be if the clutch was disengaging properly.
First step is to adjust the clutch cable so you have just a small amount of free-play at the pedal.

Alright, i'll have a look at the tightening nut and see if adjusting it helps.

Try checking the clutch spigot /pilot bearing which is in the end of the crankshaft where the gearbox mainshaft centers. When it fails or starts to fail it has the effect of gripping the mainshaft, preventing it from slowing down when the clutch is depressed and not allowing the gear to engage. Its most obvious when attempting to engage 1st or reverse from stationary, once you are rolling the gears will shift relatively easily. Don,t ask me how long it took to find this problem !!

Ok. Well this does make a lot of sense considering i couldn't change that pilot bearing once i did the clutch for lack of a tool small enough to extract it. I had ordered that bearing and have it though.
I'll definitely investigate that path although i'm not eager to drop the box and clutch again lmao