1981 Pontiac with a SBC and non lockup T 350

Suddenly, without warning, driving in very slow moving traffic, my T-350 binds up when shifting into high gear.

Binds hard, probably break something if I let it continue.

Shifting the stock column shifter to 2nd eliminated the problem.

Only when shifting from 2nd to 3rd is there a problem.

Doesn't matter if I manually shift, 1-2 then D, or start of in D, high will not apply, or better yet 2nd doesn't release.

Otherwise the trans is perfect, strong reverse, no noise, clean fluid, but I'm certain it would burn up right away if left it in D.

As long as I keep it in 2nd, it drives normal so it's not a modulator, governor, linkage or kickdown cable problem.

Any input??

Thanks

Randy