Performing a "Torque Convertor Stall Test", as per the FSM, should indicate if the internal parts of the Torque Convertor itself is failing, and in what way

Be aware, a stall test does "stress test" your Torque Convertor, hence the warning to only do it for 5secs (I think)

Thanks all for your replies. Hope you're doing well in those weird time. I have not solve the issue as the stall test and did ok. Used my endoscope camera and found no loose bolts.
Will park this issue (until it parks me). Installed new leaf springs in the meantime.