Why does it vibrate and shake when it shifts automaticly into lower gear for cruise and the rpms go back down

What is your question?

It vibrates when going back into lower gear and cruising. As long as im excellerating or coasting it doesnt happen

What makes this problem better or worse?

After going a few places

How long has your 1999 Ford F-150 had this problem?

Just got truck, not sur how long its been happening