A misfire is generally worse when the Converter Clutch locks-up, Lock-up happens at high speeds than when your symptoms are happening.
The 4L60E has 2 "Mechanical Diodes"/"One Way Clutches".
The Lo/Reverse Roller Clutch & the Input Sprag.
When taking off in 1st gear with the Shifter in D4, Your relying on BOTH too hold ALL the input torque.
The Input Sprag is MUCH weaker than the Lo/Reverse Roller Clutch.
The next time the symptoms come up, Pull into D2 (This allows the Overrun Clutches too come "On" & bypass the Input Sprag), If the symptoms go away AND it's repeatable.....D2=no shutter.....D4= shutter, The Input Sprag is about too FAIL!
If the symptoms persist, Pull in down into D1, This Bypasses BOTH L/R Roller Clutch & Input Sprag, If the symptom now disappear.....The Lo/Reverse Roller Clutch has a issue.