First off, check your transmission fluid level in case the shop that serviced it didn't know what they were doing. This is unfortunately all too common with the 4L30E since it doesn't have the dip-stick fill. You can also reset your own computer by disconnecting a battery terminal for about an hour to clear the ECU. This will force several maps to rebuild, including transmission parameters.

Take a look at the link from this site to the tranny fluid change as there's lots of good pics of the major components, including one of the adapter pan where the solenoids are located (there are 3 IIRC). I also have a 4L30E rebuild/shop service manual, so let me know if you need specific tranny info and I can post it.

Definitely check the fluid level and go from there first. The adapter pan gasket where the solenoids are is a common part shared with the Trooper and Rodeo. The last time I bought one it was about $12.