Hi mate. Youre shog sounds like a manual. Are you sure its not a clutch problem.
If you source a new g/box then make sure its from the same motor ie, lwb or swb as yours. I have recently fitted a lwb manual g/box into a swb. It fitted ok, the centre mount ok, the side mount was about 6 inch out and i had to make a prop to fit.
Personally i would not recomend this as i only did it for my "total -off-roader".
Obviously make sure the eng sizes are the same as well.
If you fit g/box yourself, then get some help. They are bl@@dy heavy.
I have a 2in body lift. That's the easyest body lift to do. plus lwb springs on a swb, torsion bars wound up and 33 bfg muds. :D