Hi guys
I have a 95 Mustang Auto V8. My problem is when the gear shifts from 2nd to 3rd or 4th while not going fast 20-40 Mph the car will start shaking for seconds

then I have to go faster or slower if I kept the accelerator on the same level it will be shaking more and more.
I went to the workshop, the technician drove the car and when it starts shaking he said (100% GEAR PROBLEM

) so is there anything else can be checked before touching the gear???
Please who knows about this problem let me know ASAP.
Regards,
Sam