r/bt50

Hi everyone,
I’m hoping someone has seen this before.
I have a 2008 Mazda BT-50 2.5 diesel, 5-speed manual, part-time 4WD. It has covered only 40,000 miles from new.
The fault is very specific:
Only 5th gear is affected.
At around 50–60 mph, while maintaining speed or under light load, the gear lever is physically pushed back into neutral.
There is no crunching, grinding or graunching when it happens.
All the other gears are perfect.
Interestingly, exactly the same thing happened about 3–4 years ago. At that time I had the gearbox oil changed, and the problem appeared to disappear completely for several years.
The fault has now returned. I’ve just had the gearbox oil changed again, but this time it has made no difference.
Before I authorise the gearbox to be removed, I’m trying to establish whether this is a known fault on the BT-50 / equivalent Ford Ranger gearbox.
Has anyone experienced these exact symptoms?
If so, what was the actual cause?
5th gear retaining nut?
Selector fork?
Synchroniser hub/sleeve?
Worn engagement dogs?
Something else?
I’d really appreciate hearing from anyone who’s actually repaired one, or from anyone with experience of these gearboxes.
If anyone has workshop manual pages or technical service bulletins relating to this fault, I’d be especially
grateful

Thanks very much.