Torque Converter Shutter???

[crawdad]

I've got a vibration that I notice when I start up my truck and before I put it in gear. It feels like it is coming from the transmission, but I'm not sure at this point. I've been told it might be the torque converter. Is there any way to isolate the torque converter to see if that's the cause? What should I check? I've checked the u-joints and they look and feel good. I've also looked at the harmonic balancer and it looks ok as well. The vibration does seem to go away when I get the truck up to speed. The only time that it really gets my attention is when I'm idleing in park. Thanks for any suggestions. BTW, this is the '86 and it has a 700R4 tranny.