Hi,
If the e-ring is item 35 in the link below (10mm), it might not be too big of a problem.
Hopefully, the shift rod didn't slip down into the case and the pin at the bottom of the shaft didn't dislodge from the shift plate.
As long as you have all the parts that may have jumped off along with the ring and you can see the slot in the shaft that the ring snaps in, you can just try putting the ring back in place and then stack the components as you see them in the diagram.
If the pin at the shaft bottom came free of the shift plate (item 6 in link below) you may need to pull the propshaft assembly to get it put back into position or the transmission won't shift.
While you see lots of parts and it looks intimidating, all the stuff on the propshaft (item 9) stays on the shaft EXCEPT the shift plate (item 6) when you remove the two bolts (item 12 x 2) and remove the propshaft holder (item 10) along with the shaft.
TIP: Have the outboard tilted slightly UP before pulling the propshaft holder assembly so that the shift plate and forward bevel gear (item 7) don't fall out of the case. However, if they do, it's not a big deal. Just make sure they're clean and slid properly back into place as per the depiction. It PAYS not to do this work in the dirt or grass.
With the propshaft and it's attached components out you can access the shift shaft and position it to replace the e-ring and position it's pin into the slot of the shift plate.
Hope this helps and good luck.
