This problem can be caused when the selector mechanism is actually engaging two shift forks at the same time. This happens when the shift forks were not set up perfectly to begin with and as the shafts and forks and selector wear interference can occur; the box has to come apart to correct!
The gear box should be checked with the linkage disconnected to be certain that the linkage is not at fault sometimes the strange knuckle joint mechanism at the end goes out of adjustment and binds up before the selector rod moves into position on the 4/5 shift fork thus making shifting into 4/5 impossible; hopefully this is the problem as repair is just a matter of adjustment with no removal required
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