It is common to have a 2nd gear synchro that feels like it is working perfectly suddenly feel like something broke and immediately grind. Due to the increasing age of these vehicles the inner 2nd gear synchro rings are starting to fatigue and break. This usually doesn’t happen unless the trans has seen many hard 2nd gear shifts at high RPM, but it happens. Here is a pic of a broken inner 2nd gear double synchro ring:
