Saginaw Transmission/Clutch problems

[Sicboy]

I have a '66 Chevy long be pickup with a 327 and a saginaw 4sp. About two years ago I had the tranny rebuilt at the same time the engine was rebuilt. After the tranny was installed, with a new clutch, throwout bearing and roller pilot bearing, I was not able to downshift into 2nd...it would grind. Everything else was fine. Three months ago, I pulled the tranny and took it back to the mechanic that rebuilt it. He said that the synchro had broke. He repaired it and I reinstalled the tranny. It worked great for the past three months, until today. I was at a stop light in neutral and when I tried to shift it into 1st gear, it would not go in at first. After a few tries, it finally went in. When I pulled away, the clutch pedal began vibrating very loudly. When I pushed the clutch pedal, the vibrating would stop. No problem shifting into 2nd. When I tried to shift into 3rd, it would grind a little. When I shifted it to 4th, there was no 4th. After a few throws back and forth, 4th would finally engage. Every time I try shifting into 4th, this happens. No matter what gear it was in, the clutch pedal vibrates very loudly. When I depress the clutch, nothing feels out of the ordinary. What could be wrong?