Ranger Mods

I have had an issue when reversing in L 4 on the last weeked:

I was in Low range reversing up a little grassy hill, just on idle.
It felt like it locked up or I had ran against a obstacle and building pressure, then released, gave me a kind of strong rocking motion.
Back forward did not help.
Back in high gear and left it for a few hours all good. It was a hot day on, but still.....?
I kind of thought it felt like the brakes where kicking in for hill start but why?
I don't believe it is the gearbox, as this kind of problems would IMO not simply go away if is coming from there.
Any other thoughts by anybody?I

I read that thread last week https://newranger.net/forums/viewtopic. ... 63#p141263 and Murphys law hit me on the weekend
Also I found on other forums that the issue is not unheard of but so far nobody found a root cause or a fix.
It seems to hit manual and auto alike and starts with PX1

Any suggestions are highly appreciated as the cars reaction is very disturbing

Mine is manual 2015 BT50, PX1 equivalent

Cheers
Mike

No ideas anybody?
Cheers Mike

Nope.