You will have to drain fluid and drop the pan to get to the solenoids.
For me anyway, whenever I have had the solenoids replaced, because I was beginning to see shifting problems, my transmission has gone out totally soon afterwards.
But, that has just been my experience.
I bought my 2004 EXT with 116K miles on it.
I had to have the transmission rebuilt at 125K miles.
You did not mention your mileage, but if it has normal mileage for it's age, you are probably in the most common mileage range for failure.
Some go more miles, some go less.
Every GM transmission I have ever had rebuilt was between 75K and 125K miles.
When my transmission in this truck began to act up, the fluid looked good, but it had a slight burnt smell to it.
The final failure I had was the truck would go in 1st, but it would totally slip when going into 2nd, letting the engine free wheel to high RPM.
If you let off the gas, it would finally shift into another gear long enough for me to drive to my local mechanic's shop.
For a cost of around $2000, I had the transmission rebuilt with regular parts and also replaced the torque converter.
That was 6 years and 100K miles ago.
The transmission in my EXT is supposed to be the more heavy duty 4L65E version, with some upgraded internals over the 4L60E, but it failed just the same as any other transmission I have ever owned.
I've had the transmission fluid changed by dropping the pan and replacing the filter twice now. (Once because one of the cooler lines had popped off the side of the transmission and left me stranded on the side of the highway until I was able to get the truck towed to the shop to fix the line and change the filter and top off the fluid.
)
So, you may get by with replacing the solenoids, but I would still be setting some money aside for an upcoming transmission rebuild, just in case.