My ATR tranny suddenly started having problems after I picked it up from the muffler shop.

1st and 2nd gear pops out of gear and they also grind going into gear at any RPM. The only way to get it in gear with out grinding is if I work it in very slowly.

All the other gears are perfectly fine and all the gears were fine before I dropped it off.

The clutch seems to be fine. I bled it just in case. I haven't checked the cables yet.

What to you guys think it might be? Retainers and syncros? Clutch? Cables?

Thanks in advance.