Someone else posted this same problem on this site, check back a couple of days on this site.
It may not be the sending units.
As I offered to his post, This same thing happened to my 87 Sammy. It was that the fuel & water temp gauges SHARE a common gauge "Choke" (which is a voltage reducer) crapped out.
A replacement dash from a Junk Yard is the easy answer.
CAPT