I just got a ranger and have noticed that under harder acceleration the transmission is sort of clunky coming off the line. It shifts pretty good through all gears but only seems to act clunky from a dead stop with harder acceleration. Starting off real slow I don't notice the clunky feeling. Checked the automatic transmission fluid and it was a little low. I added some, but doesn't seem to help. Is this normal, if not could you suggest an additive or something that work the kinks out. Oh yeah the color of the transmission fluid looked ok, not real bright red but not real brown or nasty.

1999 Ford Ranger Stepside XLT 4x4 Auto

 

That is the infamous slip-yoke.

Use that as your search term.