You can check the pin by removing the rubber boot around the lever and trying to drive out the small pin through the shaft with a punch. If it won't drive out then that is your problem.
To check the weld as Mike suggests the the gearbox top has to be removed,put the main gearstick in neutral, take the cab floor out, and remove the bolts all round the gearbox top cover, then lift it straight up, you may have to take the gearstick off first, I can't remember, then turn it over and examine the relevant parts.
When you have repaired whats necessary make sure that all gearsticks are in as far as possible the same positions as they were when you took them off and lower the top gently into position.As Renze said , allow 2-3 hours at least to do the job.
Dave