The sending units have become painfully difficult to find... The stealership I called said both the front and rear diesel units have been discontinued from the 83-91 model years entirely. I was told a rear unit could be had for close to $500 for my 94, and the front was obsolete.
Strangely enough they had gas sending units listed and available...
Seems as the cost and complexity of the newer diesels keeps rising, the parts for the older ones "convienently" become obsolete.
On my white truck I experimented with a single turn potentiometer (cant remember off-hand what the ohm range was) with lousy results. The sender worked very well for about 1 week then the diesel ate up the windings inside the little *******.