So yesterday i started getting a loud tapping, dull metal on metal noise randomly when driving in 3rd gear, it was only really noticeable going UP hill, and under load.
Today the noise started in first and second gear as well and was extremely loud, then gear changing became very difficult.
Am I right in assuming the gearbox obviously needs a rebuild? I have a spare matching LT77 (Its a 1992 200tdi 90) so intend to swap this over.

Following on from that - does anyone have a good guide for swapping the gearbox out on a driveway, I have hand tools and a trolley jack and railway sleepers to raise the truck off the ground.

Hard to diagnose without experiencing the issue in person, however, worth checking that the oil is at the correct level and perhaps drain the oil and look for metal in the oil/ on the mag drain plug before removing your current gearbox. However, from your description, it does sound like there has been mechanical trauma in there so a change may well be inevitable.

When I removed my LT77 for rebuild, I removed the seat box, floor panels and transmission tunnel (nuts and bolts) and removed the transfer box by hand, from above. The gearbox was then easy to remove, again by hand, from above. Bell housing bolts are also easy to reach if you remove the tunnel. You can remove it from below, but be very careful as the gearbox and transfer box are very heavy items.

Check the mainshaft is in good condition on your replacement and if not already fitted, fit a cross drilled gear in the tansfer box to ensure the mainshaft is lubricated.

Use a transmission Jack or engine crane to avoid injury to yourself or the boxes

As said tfer box out first, remember to chock wheels as handbrake is on the tfer box!
Not a hard job at all, but it is a very dirty job.
Prop nut removal tool (3/8 version) is pretty much an essential time save.

I took the seat box a floor out mine, lowered it down with the engine crane an then dragged it out.

For all the time it takes to remove the seat box, it's totally worth it.

Thanks guys, got everything out with just a jack and rope. the t box is pure evil@@ Their must be an easier way.

I took pics so will post a how i did it up later. just ffing about with the diff lock now.

Yup there is.... an engine crane through the passenger door & no need to remove the seat box.