my 2001 blazer with auto trans 4 speed, slowly moves on take off in 3rd and 4th but will take off in 2nd and first then i manually shift into 3rd and forth.
My guess would be the trans selector switch is either going bad or is badly out of adjustment.
Or weak shift solenoids in the tranny. PCM can't kick it down but the manual valve body still works! I think they can be accessed via the pan.
Search Chevrolet Blazer Questions
Chevrolet Blazer Experts