96 Transmission shudder.

I have been getting a shudder in my 96 Transport transmission while shifting from 1st to 2nd gear. You can barely feel it when the engine/transmission is cold but as the engine warms up it shifts very hard. To the point of actually shaking the steering wheel. It only happens from 1st to 2nd and other than that the transmission works great. I had the transmission back flushed, fluid replaced and a friction modifier added but there is no difference. Anyone else ever have this problem? If so is there a reasonable fix?