Auto Trans issue

[3KGTVT]

Last August i had my transmission rebuilt after it completely died. When i picked the car up the mechanic told me "Only use overdrive if your driving above 50MPH" You guys ever heard of that? Wtf? Anyways. I didnt drive it in the winter at all, so I barely put 1000 miles on the new tranny by the time september came around.

But now I think theres something else wrong with it, or maybe its just normal and I just realized it. First off, on the way home tonight I noticed when im driving 35-40MPH and let off the gas, the RPMs dip to 1600, steady out for a second or two, and then rapidly drop to 1k. Normal?

Another problem. If I dont turn OD off when I am braking, the tranny seems to "jerK" when it downshifts under 20mph (if I do turn OD off, it downshifts without an issue). Normal?

The tranny came with a 1-year warranty, should I bring it back before it craps out on me again?