I'm thinking input shaft in the tranny.  Had the same thing happen with my 5spd in my 81.  The gear on the input shaft lost a tooth and sounded the same as yours. You've diagnosed your issue pretty good...it is a direct drive and because it's not happening in 4th it's an issue with the input.  You could rebuild it if you're good with trannies but rebuild kits are really hard to find, otherwise go with the Marlin HD tranny or a bone yard one.  The Marlin HD has a beefed up input shaft for our trucks and it'll take a good beating.  I ordered one a while ago and it's pretty solid!