Rick, Thank very much for the info. I think you are on the right track, as I'm pretty sure this is a mechanical not electrical problem. My starter is an MAW-4013. probably the original starter. Not many '37 Plymouth pickups around, so not many shop manuals. I've been looking for one for at least two years. Anyway, turns out the '37 Dodge pickup uses the same starter (probably other similarities as well) so I've order a '37 Dodge pickup shop manual.
Meanwhile, pulling the starter is probably old hat to most of you guys, but it's a big deal for me. First step is to detach the foot peddle shaft from the starter motor lever. Looks like a riveted connection. Not sure the best way to do that and really not sure how to replace.