1999 chevy silverado 4l60e trans problem

[84Chevy]

Trans was rebuilt a year ago ... when its in gear its strong... my speedo goes crazy and it wont shift out of 1st... one minute it will the next it wont... or if your cruising down the road it will down shift to 1st..... sometimes itll do it others it wont... the trans shop said speed sensors... i replaced the  one he said it was ... still does it... took that old sensor i replaced and swapped it with one of the other speed sensors... still does it....should i replace all 3?  i plan to take it back to him some time this week... just want other opinions.

[BIG DOG]

i thought that black control box  on the shifterat the trans      does all that  ?