clutch is about $100.. tranny rebuild is about $1000, depending on problems.
I'd recommend getting one from a junkyard.
sounds to me like the differential is FUBARd in the one you have, which usually isn't touched in a tranny rebuild... a new diff is $$$, so you'd be better off with a junkyard tranny.
First though, I recommend pulling the axles out and checking them. make sure both axles are inserted completely and properly into the tranny. Nissan only made about 24 different axles for these cars and I've seen a lot of problems with not getting the correct ones (1/2" shorter or longer, splines the wrong size, all kinds of crap like that.)
This may be the case on this car as well. let's hope not because it could have done some nasty damage to the tranny.. That would also explain why it's out of fluid because it leaked out of the seal around the axle.
I would also try to find out who last worked on it- if she had someone replace the axles with the wrong ones in the past, or not get the axle seated in the tranny correctly, then this could be the result...
As for dropping the tranny, it's not hard at all. disconnect battery, remove starter, remove clutch slave cylinder, remove shift linkage, unbolt bellhousing bolts, unbolt tranny mounts, support tranny w/ jack, pull out tranny.
if it's got a stock intake, it will be much easier to do if you remove it. probably removing the Y pipe will help as well for a first-timer, but neither are absolutely necessary.
usually takes me about 45 minutes on my car...
here's the writeup I did on it several years ago...