GM transmission, the 4L80 E unable to select 1, 3 or 4th..

I would check all the electrical connections first. Unplug and plug them back in while looking for any corrosion. They should all be weather pack/deutsch connectors but failures happen. Also remove all the fuses and push in relays in the power center so as to establish fresh connections.

DO NOT use wire brushes on the face of the connectors as voltage may be inadvertently sent to other terminals. Look up Mexicowanderer posts for a good electrical cleaner that he has recommended.