Synchros?
 

How likely is it that both my 1-2nd gear synchros and 3-4 gear synchros went out at the same time? The transmission has the problem where i have to be almost stopped to get back into first and it now grinds (sometimes lightly, sometimes a little harder) going into every gear but reverse and 5th. When I double clutch it shifts better, but i dont have to rev-match to get it to shift fine, the RPMs just fall to 700 and it shifts fine no matter what the speed is. How likey is it that i just need tranny fluid? The problem started lightly with third gear and now all the gears do it on every shift. Synchros arent something I want to tackle, but where could I buy them if i have to. I searched autozone's website and they dont have them. Neither does Courtesy Nissan. Do I have to go to the dealership?

first gear lockout... dont force it

have ya checked the clutch master and slave cylinders?

bennieb has a good point. If your clutch isn't disengaging properly, grinding gears could be a sign of this. It makes a sense that your reverse gear isn't griding because the reverse gear is never synchronized. Not sure about 5th though, I'm not a mechanic.

have you changed your gear oil? my 240 had been doing the same grinding i went to the sports car only shop by my house and they sugested red line full synthetic gear oil (mt-90) i changed my gear oil and my gears go in perfectly(these 240's are prety old and alot of people neglect to change their gear oil)

P.S. changing your diferencial oil might be a good idea 2

Yeah I bought some royal purple, but my drain plug is stripped... i gotta get on that