Transmission shimmy
 

I have 98 GMC K1500. When I am driving up a grade, when the transmission is supposed to down shift it wants to shimmy for a few seconds before it finally shifts. That is the only time it does it. I can push the accelerator to the floor and back up real quick and it will down shift better. Taking off it will shift normally through every gear. I have already changed the fluid and filter, still does it. Please help. Thanks