"Wind up" in reverse

BEFORE I start investigating I thought I would put this here for your thoughts/consideration.

BigJimny has:

1. Jimnybits Rear Disc conversion
2. ARB Air locker
3. 3 inch Spring lift
4. "Castor corrected" rear arms (ok, I know its not castor on the rear, its diff nose angle)
5. 16" Vitara wheels with Maxxis Bighorns
6. Rocklobster 4:1

I have a weird problem, she will not reverse properly sometimes on gravel surfaces.

If you enter gravel from a smooth surface, in reverse and slowly, it will cross the gravel just fine.

If you STOP on the gravel and then either enter reverse or continuing the reverse you were doing then within 2 feet it stops as if it is totally wound up , ie the engine will struggle to move the car, the rear of the car rises as if the rear brakes are on and its pushing against them. Pulling forward and then slowly reversing again will often clear the problem.

Has anyone seen anything similar and can point to where to look first. My first point of call is the rear brakes, I am thinking that the handbrake cables are tight and perhaps the movement of the axle and arms pulls the brakes on.

However if someone out there can say "I know what that is" then that would save me looking.

And before you ask the "what was the last thing you changed before this started happening" - it had a complete rebuild so could be anything.

Oh, and note "Gravel" not tarmac for some reason

My old sankey trailer was a git for this. Even with the brakes override on it wouldn't reverse turns out the tiny bit of movement before the override was enough to make the disks grab.