Those boxes are not really up to task, and if it has been used to tow or done sand 4x4 work especially in high range without a cooler AND never had a fluid change I cannot say I’m surprised.
If it was mine I’d try and get a used box and get it right and replace it with another Hilux with an A750F box that has been looked after, which means I think either a diesel soon after yours or any V6. The trouble though with any used part that is a known weak link is good used ones are expensive, hence why these aren’t cheap.Very much why Toyota B diesels or Nissan Z30 diesels are/were expensive used, opposed to Nissan Patrol diffs and transfers where wreckers had just about every one they ever wrecked still on the shelf.
There will be boxes around, just need to find a wrecker with a few. Have you tried these guys?https://www.gumtree.com.au/s-ad/malaga/ ... 1126788285