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The range switch on the outside of the transmission is the prime suspect. The problem could be the switch itself or the connector. Be aware that the connector in this vintage used a viscous sealant compound to help keep moisture out of the connections. It is not unlike hot melt glue. Warming the connector with a hair dryer or heat gun on low heat will help soften the sealant and allow easier disconnection. Without softening the sealant it can be almost impossible to remove the connector without breaking something.
Coincidentally, I just wandered past Tomah from the northwoods yesterday instead of fighting the traffic today. I'm sure you know how that can be.