Little background.
My mom bought a 95 Blazer. 4.3, 4x4 4door. Would not pass emissions had the cat replaced and don't remeber what else they tried.
So I got to bring it home with me. Did a tune up.
Plugs, wires, cap/rotor, front O2 (was old anyway) air filter, pcv valve.
Still had a miss.
Called my dad told him I was thinking the spider for the injectors. Said yeah go ahead and pull the intake. Found the return line leaking.
Now it has passed emissions with flying colors, and runs great.

But after I had done all this driving around I found it would not shift into OD.
Shift pretty much normally 1st to 3rd. Has a little bit of a rough shift here and their but not bad. Going from OD to 3rd and back to OD with the shifter does nothing.
No check engine lights, passed emissions with this issue (they checked for codes) Just put in a starter today going to drive it a couple days and check for codes to be sure.

I am thinking it's something in the valve body. But I haven't been to far into a transmission or rebuilt one. So my question would be what do you guys think?