Shifts slipping th350

[JooseyJay]

Hey all, was wondering if anyone has run into this same thing.
With trans in D my 1st and 2nd just sounds like 1 long gear at part throttle. (slips pretty bad but accelerates almost normal)
Full throttle it shifts good.
If I manually shift from 1st to 2nd it shifts fine.
Can't find a leak in the vacuum line to the modulator, fluid is good, and I'm not exactly sure if the modulator itself is good.
From what I've read online auto shifts slipping and manual shifts good means vacuum leak/modulator issue or trans is going. Something about using separate circuits in the valve body.
Anyone had the same situation? Have a feeling I'll be pulling the trans soon...