Swivel pre-load. The front hub assemblies turn/swivel (for steering) on taper roller bearings, below and above the swivel ball. The taper bearings are shimmed to pre-load the bearings.
When the bearings wear, they become loose, which adversely affects the steering/handling. It also affects seal pressure/alignment on the swivel ball, leading to oil leaks.
If the bearings are not badly worn, but pre-load is too free/loose, adjust thickness of shim pack under the top swivel bearing cap. If wear is bad, replace the swivel bearings.
Wear of bushes in the radius arms (2 at axle end, 2 at chassis end) is a common cause of your symptoms. They can look ok at a quick look, but like all suspension link bushes, wear is hard to detect from a simple visual check, which is often deceiving (hundreds of people have been caught by worn bushes that look ok, but obviously are not when the arm/link is removed). These would be my 1st guess as to your problem.
Wear of bushes in trailing arms for rear axle - when worn, misalignment of the rear axle can steer the vehicle.
Bent trailing arm or chassis mount for trailing arm causing misalignment of the rear axle. Trailing arms bend it they land on rocks, etc. The chassis mount can bend forward if the rear wheel is driven (in reverse) into an obstruction.
Panhard bushes ....
Tierod ends ....
Slack in steering box ....
Edit: discos should have very good manners driven hard on dirt roads. However, I had different tyres on my disco on the last 2 occasions that I drove back from Birdsville via Windorah. The difference in acceptable speed was marked between the tyres that were fitted. But this was not an issue with pulling to one side, just the way the tyres handled when driving on loose, marble like gravel roads.


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