Tranny fluid changed, now reverse not engaging
2004 zts 2.3L. We are around 93k miles and had a shop change the tranny fluid, they dropped pan, cleaned magnet, put in new filter/gasket yesterday.
Driving it all yesterday was fine, shifting was noticeably smoother, however this morning when I put it into reverse it didn't engage, it was like it was still in neutral. When I gave it some gas it just sat and rev'd up an additional 500 rpms or so then would kinda engage and stutter back, very jerky, like in a manual when you're letting the clutch out to soon or don't have enough gas.
All other gears seem ok, though shifting wasn't as smooth as yesterday it seemed. What can cause the reverse to not engage properly? What with the tranny fluid change could trigger this? I did some quick searches and found 3 possibilities if anyone can shed some light, bad oil pump, solenoid, intermediate band.