Sounds like a bad syncro or blocker to me.
Is 2nd to 3rd the only time you hear the grinding? No other gears? You may try to adjust the clutch before doing a rebuild on the
tranny. The general rule of thumb is you need a 1/4" gap between the face of the throw out bearing and the fingers of the pressure plate when the pedal is all the way out at the rest position. You can check it by removing the metal clutch fork cover down on the bell housing. Over time, the disc wears whcih pushes the fingers outward and the cable itself stretches, causing what you are describing. If this all checks out, then a rebuild is in order.
PM sent......
Richard
Tech Support
Tremec TKO, T45 & T56 Transmission Systems
__________________
Self Proclaimed "Resident Manual
Tranny/Driveline Guru"
Go with PI heads and quit watching The Fast and the Furious.
(thanks 2 IhaveadopeGT)
Remember, it is almost impossible for anyone, myself included, to diagnose a problem over the internet without seeing the car. My advice is simply that, advice based on years of experience. I don't want you or anyone taking my word as gospel. I am trying to give some direction as to what I think may be happening.
Transmission question/problem 
