i have a 96 wrx 4 door and i think its may need a replacement gearbox.
jumps out of 3rd and 4th, also looks like its leaking oil, havnt had a good look to see where the oil is coming from but gearbox oil is low.

can anyone tell me the difference in gearboxes. is it just ratios?

ive seen one from a 94 5 door and looking at this only one gear has a different ratio.
http://www.catherineandken.co.uk/sti/trans.html

my gearbox code is TY752VB4AA

but would it fit and would it be a straight swap?

 

You need to get one with the same physical fixings, ie 4 bolt/ 8 bolt. You also need to get one with the same final drive ratio, ratios is down to personal preference or what you can get your hands on at the time.