1997 pontiac trans problem

I have a 1997 pontiac grand prix with a 3.8 liter v6 engine, and my trans occasionnally seems not to want to shift from 3rd to 4th it eventually shifts but its later than it use to shift. I was thinking that it may be
the shift solenoid,if so can it be replaced by just removing the trans pan or do the trans need to be removed from the car . Also is it just one solenoid or are there several. If the trans do not have to be removed do i just drop the pan and replace the solenoid. thanks for your help.

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