It looks fine to me, except the ablative of "duo" is duōbus. It's also much more common to put numbers before the noun, so I would say "ā duōbus servīs". Or alternatively you could just use the active voice and say "quam duo servī vehunt".
The gender and case of the relative pronoun is the same as the noun it is taking the place of.Lectīca ā duōbus servīs vehitur.
Here "lectīca" is clearly the subject, so it takes the nominative....in lectīcā, quae ā duōbus servīs vehitur...
So "quae" takes the nominative too, and it takes the gender of "lectīca".Duo servī vehunt lectīcam.
Now "lectīcam" is the object, and therefore accusative....in lectīcā, quam duo servī vehunt...
So now "quae" takes the accusative to become "quam".
Is that clear?