When going uphill, there's a "thunk", as it lugs down and needs a lower gear

Hello, I would suggest first having the mounts for the engine and transmission checked, as broken or worn mounts will cause the drive train to move significantly under load, possibly causing the thunk you are experiencing. Here is a link to some shops near you that could inspect this for you! http://repairpal.com/ford-within-10-miles-of-loyalton-california-96118