1970 THM 3rd gear no pull-troubleshooting help needed-C10

[337]

1970 c10 stock with original turbo hydra-matic w/ 4bbl v8 350 engine.
Trouble shooting tips needed.

Was on open freeway trying to travel 60-65 against high speed head winds.
3rd gear stopped pulling. truck started vibrating.
Coasted until truck downshifted to 2nd on its own. then vibration wasn't as noticable.
sped up and truck would shift to 3rd again on it's own.
as soon as it hit 3rd, no pull and heavy vibration comes back until it down shifts to 2nd.

Manual shifting to 3rd also has no pull.
2nd and 1st i still got.
fluid is fine.

doubt tranny was every dropped for maintenance/rebuild.
86k original miles.
truck sat in garage for 10 years before back on road last year.
put 2k easy miles on then this happens.

[PGSigns]

Time to drop the pan and see whats in it. I suspect you will find a bunch of stuff in it and looking for a rebuild.
Jimmy

[AussieinNC]

Symptons detailed point to Direct clutch failure....

Make sure the oil level is truly correct...Hot, idling in Drive with park brake on...check oil level, should be on Full Hot mark.

These trans are good but everything wears out sooner or later...