Transmission shifts

Hey tw! So I have 05 taco v6 auto transmission with 86000 miles. so when going to 5th gear from 4th it shift weird .. so here a example im in 4th gear the truck is ready to shift to 5th my Rpm are about 2300-2400 and then when it starts shifting to 5th the rmps go to 2100 stop like if it's in5th gear but then rpm drop to 1900 and then it's in 5th gear .. it's not rough when it does it but it noticeable.. it happens when I'm accelerating the car smooth but if I'm merging into the freeway it dose t do it just goes into 5th like it'suppose to .. I just did the flush 3 months ago

 

Sounds like you described the normal shift to 5th then lockup.

 

And what's causing that?