Don't be so quick to now blame the starter. A little diagnostic work is
in order before getting that greasy pig out of there.

According to Bentley for 81, there are 2 wires going to the same spot on
the starter solenoid -a large red/black from the key switch and a small
blue/brown to the fuel pump relay. Try unplugging these one at a time to
see if the starter stops turning. Let us know what you find out.

Do you have a volt meter handy?

Mark