Wow - not a good situation
Is the display defective or is there a problem with the sending unit? Perhaps they are telling you the electronics that provide the logic for data collection and display is defective.
I like the idea of going to a Silverleaf system assuming the electronic engine control isn't impacted by your defective part. Your engine might be old enough and isn't dependent on a computer to run properly. Silverleaf might not be able to interface with your '98 engine - best let them give the word on that.
You might also try the Cat forum to get some better engine advice and I would try another Freightliner dealer for a second opinion with that much money involved.