My 96 Pantera had the same problem - the starter would spin but not engage. The battery charge was just a tad low. I charged the battery and it works fine.
The solenoid passes voltage to the starter using the key. If the starter spins, it is not the solenoid unless there is corrosion on the terminals. Check the circuit for corrosion that could be lowering the voltage to the starter. FYI, the solenoid is a White Rodgers sold at auto parts stores for a lot less.
I also had an ignition switch that was full of dirt that would not pass voltage to the starter. In that case, the starter did not turn nor was the solenoid getting any voltage.
Once a year it pays to put just a drop or two of oil on the bushing from which the gear extends on the starter. If you don't, after a while, it will be rusted solid.