I had the same problem with my car, exactly the same, I was driving down the highway, tried to shift from 4th to 5th, GRIND, what the f**k, back to 4th, GRIND, oh shit.. pull over... (this was on the drive home after buying the car) so, I replaced the slave cylinder as it appeared to be leaking anyway, no luck, so I rebuilt the master cylinder (pain in the ass, cotter pin took 2 hours to get out) still nothing, so I take my car down to the shop on the corner, my clutch was in about a thousand pieces inside my tranny, though my problem was the worst case scenario, it could be your slave or master.. I hope it is for your sake... check your clutch fluid level, if it's abnormally low, you probably lost all pressure in the system, therefore you can't disengage the clutch to put it in gear while the engine is running, maybe try putting it in gear with the engine off, take off the e-brake and see if you can move it at all.. if the clutch is shot totally it might roll, if it's the hydraulics then it won't, just an idea.. anyway.. sorry for the long post.. it's late and I'm bored