Sometimes, when I try to start my 1996 GMC it will click once but won't start. Truck can sit for a while and might start. Problem is erratic. Could this be the starter solenoid? When truck does start it runs great. Battery is good.

. Remove and fully charge the battery. Most auto parts stores will do this for free.

. Replace the starter solenoid, or complete starter motor, if the problem persists.

Visibly and physically inspect both ends of both battery cables. Make certain they are clean and tight.
The Positive cable on these batteries are the most likely to have issues: corrosion here can cause many problems.
On some configurations, there are actually two cable ends and a lead spacer on the Positive connection.
With the cable(s) disconnected, inspect inside the Red protective end by probing with a stout pick or small screwdriver. The cable's surface should be shiny and clean.
If the cable eyelet is badly corroded, you may need to replace that cable.