I call it the shifter plate. Don't know correct term. I mean the whole top piece that unbolts. It houses the shifter & the shift fork inside. I did this on an SM465 which is basically the same tranny. The rod on the spring that keeps the shifter in the 1-2 side was wallowed out & would stick, not allowing the shifter to push over to the 3-4 side. It had over 300k on it though. The synchros were still good. If your grinding going into gears but can still shift OK, than it might be the synchros (assuming clutch is good). Someone will correct me if I'm wrong.
Scott