Is it something as simple as the gear shift lever rotating on the shaft? My FJ is like that (though at 90,000 miles it's allowed some wear!) and I simply have a huge nut and bolt on there.
If the problem is inside the gearbox, then it's a slightly different matter. You'll need to remove the gearbox and replace whatever it is that's bent or broke inside - and I'd suspect it's the selector lever. Not too hard if you're confident with spanners, as the whole box comes out in one go without splitting the crankcases.
Have a look at that shaft first though...
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