When moving the gear selector into drive position, the dash illuminates intermediate gear?

When moving the gear selector into drive position, the dash illuminates (Intermediate gear) and the car drives in that gear. It shifts hard, I believe because I’m driving in normal conditions (not towing a load or straining the engine on hill climbs, etc.) and is definitely putting strain on the engine as it over-revs and is locked out of OD. I’m planning to do repairs myself as I’m an experienced (at-home) mechanic, but I’m not sure where to start.