There is no guarantee that the "New" starter from Shuck's was any good to start with. It was likely a rebuild that may have only had the contacts replaced before it was shipped out. You might cruise by your local wrecking yard and pick an oem one up. I picked one for only $10 because it wouldn't even spin, I figured would use it as a core. Just to check it I pulled the cap and the contacts were worn, replaced them and it has worked fine since.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed