OP, your problem sounds somewhat like the feeling I was getting when my trans started having issues, but not quite the same.
One tell tale that is very significant, your having issues in all gears. This strongly suggests it's not the actual gears. I would definitely pull the rear diff and check/service, pull the clutch, and check the output sections warp mentioned.
To me it doesn't sound like a detent spring. I'd lean towards the clutch side of things in this case, the "slip" feeling you have in all gears can only be a few things, most not involving splitting the cases.
After reading a lot of bike problem threads doing research, nearly every time the R3Rs have significant issues it's because someone, somewhere, didn't assemble something correctly.