Steering Box

Try looking at the condition of all the steering rod ends, wheel bearings, shockies, even tyre pressures before getting too involved in the steering box. More than likley to be something simpler & less expensive than the box.

The steering boxes do wear but most can be adjusted to remove most of the slack.
Jack the front up and use stands to support the vehicle so that both front wheels are off the ground.
On the top/side of the box is a nut with a grub screw in the centre, probably around 14mm from memory, some have an allen head. The nut is a locknut.
Before loosening the nut mark the steering box case, the nut and the screw so that you know where they were before you start fiddling.
Carefully loosen the lock nut, then slowly wind the grub screw in checking the steering every so often to see if it is binding. As soon as the steering wheel starts to bind back it off a whisker and tighten the locknut.
Keep note of how many turns you wound the screw in so that you know how much you have adjusted it.
Take it for a drive, make sure the steering doesn't bind and self centres as it did originally before adjustment.
If the box is badly worn you may have to adjust it a second time to get the slop out.
If you can't adjust the slop out then a rebuild will be required.

Check the rubber joint on the steering shaft. Inside the car from memory on those.

Roughneck. I have 2001 100 series and have just had all the same problems you talk about. The steering slack was just getting to be very annoying and I had to constantly correct and over correct the damn thing especially at cruising speed. I also noticed that the slop got worse after the box warmed up. My first thought was that it had something to do with the fluid so I took it to Eastern Suburbs Power Steering in Bayswater Victoria. They took the box out of the car to adjust but couldn't get all the slop out so they did a rebuild! The rebuild cost $880 all up including align. I thought this was a lot of dosh but couldn't stand the feel of the steering so I got the job done. The steering is much better now but I think I still have some slop in the flexible joint that is just inside the car at the fire wall. I will fix this when I get back form Wanaaring in a week or two. Sorry, but a box rebuild could be the best shot for you. The ball and Nut design qwill always have some slack. They are tough boxes but a little lacking in feel at the straight ahead. Ken