Engaging into first causes crazy vibration

[clean2g]

I'm having some serious vibration when engaging into first from a stand-still. Every other gear seem to engage fine and tranny feels fine, although some gear engages on the highway weren't as perfect as usual. The vibration also goes along with a rattle. Tranny was rebuilt about 15,000 miles ago by Shep, ACT 2600 with approx 18,000 miles on it, new clutch slave and master and ss clutch line. Car is a 97 GSX with 119,400 miles.