Manual Transmission - 3rd GEAR GRIND!!!!

Ok well I know this is common and I just want to check with you guys. When I shift in to third gear at 5500+ RPMS it will grind alittle bit, and when downshifting to 3rd at speeds over 50 MPH it will grind a bit. Now my dad says its the syncros but from what I have read it sounds like the slave cylinder. Keep in mind my car is a 98' (with the crappy slave cylinder GM doesn't even make anymore) with 106k. So which one is it? If it is the slave cylinder how do I go about accessing the old one to replace it? Do I have to drop the tranny? Also my clutch fluid is black could flushing the system fix alot of the grinding? Thanks Guys!!

ok i'm new to manual cars, but i can say for SURE the fluid should NOT be black. In fact NO fluid in your car should be black. If it's black CHANGE IT ASAP!! It wouldn't do any damage to change it.

yeah I know I just realized this and I am going to change clutch fluid, trans fluid, coolant, and rearend (when I install my 4.10's) Does any one else have suggestions?? I also found it only grinds into 3rd from 2nd when I am changing gears fast, so i am thinking its the slave cylinder