Transmission Troubles

Hello I have a 99 GLS with 100,000 miles and the transmission will shutter when shifting into OD if I am real light on the gas. If I am going around 50 when it shifts that is when it happens. I had the tranny flushed and filter changed about a month ago to amsoil and it went away for that time. They updated the computer and there was no debris in the pan. Any ideas? If I hold it in 3rd gear until I am going 55 it shifts with no problem.