at what rpm should a 1987 SDL shift at?

Check the tranny fluid level?

Would Having it too full, or too low make it shift early?

mine shifts at about 3200rpms

Depends on load and throttle position. Under very light throttle on level ground it'll be in fourth gear by 40 mph. With the pedal all the way to the kickdown switch it'll shift at around 4800 rpm.

Sixto
87 300D