Err... pardon me for pointing out the staggeringly obvious here... but can you think of a scenario where a loud clunking crunching noise from the gearbox would be a good thing?
Some questions for ya:
Are you sure it's the gearbox - badly worn driving members make a load of noise on gearchange.
Have you checked the gearbox and transfer box oil levels recently?
Have you checked for play in the prop shaft UJ's? Greased the UJ's recently/ever?