4 speed problem

I have a 1969 GTS 383 4 spd with a problem. Shifting from neutral into 1st I have a severe shimmy/vibration. Once I am rolling it goes away. No problem from 2nd to 3rd to 4th. Any clues what could be going on. I have had the tranny rebuilt, new clutch, pressure plate, the bushing and bearing at the front as well as the rear tranny mount, thanks for any help or suggestions.