Ford Repair: 91 Ford Ranger tachometer, dis module, coil pack

Question
I have a 91 Ford Ranger with a 2.3L engine, 2-wheel drive.  The tachometer started to intermittently not work (showed much lower rpms than true.)  When it wasn't working the Check Engine Light would come on.  When it started working again the light would go out.  This went on for a few months.  Tachometer works, check engine light off; tachometer not working, check engine light on.  Now the tachometer never works and the light is always on.  I need to get the truck smogged so I need the light out.  
I took it to a local shop and they said I needed to replace the DIS module and the coil pack because two spark plugs on the exhaust side weren't firing.  I replaced those items (and the spark plugs, which were fine,, they were all obviously firing OK by the condition of the plugs.)  The condition remains unchanged.  The truck runs fine, but the check engine light is on and the tachometer doesn't work.
My Hayes manual doesn't mention tachometer at all, but only refers to changing the speedometer pinion gear as a servicable item.  Could changing that gear fix the tachometer?  The speedometer work OK now, although it may be a little jumpy.  

Answer
Hello and thanks for the question...I'll try to help.
Changing the spedo gear won't solve the tach problem......they are unrelated. The problem that you are having is that the signal from the ignition module isn't getting to the tach gage. I would have the vehicle scanned for fault codes and go from there, since the check engine light is on.
Hope this helps,
Erik