I assume its an Extra Cab (122" wheel base).  The King Cabs were only on the
mid-80s models, ~135" wheel base.  Likely the vehicle is 4000-4500#:

A thread with some tow weights listed:
http://www.4x4wire.com/forums/showflat.php?Cat=&Board=UBB11&Number=630848&Forum=
f11&Words=towing&Searchpage=0&Limit=25&Main=629550&Search=true&where=bodysub&Nam
e=&daterange=1&newerval=1&newertype=w&olderval=&oldertype=&bodyprev=#Post630848

You could also take it to a local "Public Scale" (look for that in the yellow
pages) and have it weighed.  I did that w/ my '85 4Runner before I had some
custom springs built.  Cost was about $8 for the service.

--
  Roger