Unable to downshift into 5th

I just got a new Dana60 and driveshaft installed on my 95 Z28 w/6-speed. After driving it home I noticed it up-shifted into 5th gear very easily. When I tried to slow down and down-shift into 5th from 6th. It wouldn't go into gear. It doesn't grind or make any noise, it just won't go into gear. I can put it into 4th then without taking my foot off the clutch put it into 5th, but cannot put it into 5th gear directly from 6th. Does anyone have any clue what could be causing this? Bad Syncros?

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