Just purchased 1990 Corvette, car shudders slightly when transmission in OD

Hello, the most likely cause is the engagement and disengagement of the torque converter causing the vibration you are experiencing. This could be due to wear and a concern, or could be due to friction modifiers in the transmission fluid being broken down. I would recommend attempting to perform a transmission fluid service first before any further diagnostic testing. If this does not help, seeing a repair shop near you for further testing is needed. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops