what year and miles is yours? Mine packed in at 70k/km 3rd was crunching (is) fourth felt really hard to get in, crunch in 1st and 2nd but that went away replacing (hopefully happening as I type this..) with a CD009 donor box that has better design and should last a good bit longer. What oil are you / have you been using? When I stuck Nissan oem oil in and a shot of.. mmm can't remember what it's called it behaved better, think I knackered mine with shortshifter / redline oil combo!
Might be better trying to get a CD009 depending on refurbing costs.
If the CD009 box is the one from the HR,then i was pretty reliably informed that these do not fit on the De engine,and the Bell housing is a different shape,so maybe worth checking if you havnt already that it will fit.
To the OP,trying genuine Nissan gearbox oil with Moly slip and see how you get on with that,as you may save yourself a fortune.