Change the starter drive- - - - -not the starter. There is a one-way clutch in the drive that slips when the engine starts to keep the armature from overspeeding before the driver can release the pedal. The clutch is worn and is slipping both ways some of the time. It was a very common failure that every mechanic worthy of the name was familiar with back when these trucks were working for a living, not pampered toys.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!