Sounds like the 2-4 band took a crap. You might just be lucky, and it's just that the servo's apply pin doesn't engage the band completely- but that's a wild chance.
Note that when you row your own (shift manually), when the stick's in 1st or 2nd gear position, the line pressure (internal trans fluid pressure) is almost DOUBLE the normal presure. So that might "mask" a problem. For instance, my trans will slip in 2nd (under light throttle) and 4th (medium throttle upshift) But if I row my own, the trans hits 2nd - and most importantly, it stays in 2nd and doesn't slip out.
-Tom P (Hot rodded 1986 Firebird 2.8l)