i have seen the speedo cable snap at the cluster end.
get underneth and unscrew the cable holder and see if the drive is turning when u turn the wheel/taitshaft.
if it was auto it may have an electronic sender insted of the cable.
not to sure but.
a silvia one will fit but that depends on your diff ratio. for acuracy.
3.7: blue
3.9: yellow
4.1: red
4.375: white?
they can be really tight getting out some times
it sounds like a coincedance but. perhaps some metal got washed up into the drive when u changed the oil. jacked up on one side etc.
hope that helps