Not going into low gear

Just got into Tuscaloosa to visit Family after putting some serious miles on my truck. This morning I woke up and went to go give her a bath and when I got to the car wash I noticed having to give it a lot of gas to get over a little hill. That made me realize that it wasn't going into Low gear. It will go into Low gear if I shift it into first gear. is there any simple solution this could be or is it time to take it to a shop? Thanks fellas

 

Check transmission kickdown linkage on the throttle body. Make sure its in spec. Also check fluid level. Check the simple stuff first.

 

like most issues with my truck it just seems to have fixed its self

 

so It went away and came back. It doesn't like to go into low gear and sometimes it will just pop into low gear as it is pulling off in second. Check Engine Light is on and a code came back for shift solenoid. Not sure where it is or how to replace it any imput would help. Thanks guys