Its the synchro. If their are particles in the tranny fluid, it is the tranny. I have the same problem from 3rd to 4th. If I hold the clutch in for a second, then it won't grind. I had 3 separate places verify that it is the transmission. With my car, they don't rebuild it because the parts are unavailable. My only option is factory new or a used tranny. I would bring it back to the place that you had the transmission done in the spring.
Did they rebuild it or put in a used tranny?
It sucks, especially since you just had it done.