Transmission shifting problem

I have a 1992 automatic lexus V8, that when I drive it the car shifts fine all the way up. But when it shifts down from 2nd to first gear you feel there is a nudge in the car, could that be the a transmission problem or something else? like not enough oil in the rear differential?

Check the trans fluid for level and condition (does it smell burned?)

well I did drain and fill the trans fluid with new oil, new filter and a new gasket, but same problem!