Toyota Repair: 89 toyota pickup 3.0 auto 4x4, rear differential, gear ratios

Question
I have DTC's in the following order,   12-14-71   RPM sig. ignition sig. and EGR system malfunction. Q. about the first two, How should I start to fix such a problem?....what should I look for? and as for the EGR, I've replaced the VSV and checked all vac lines to the EGR(good) the EGR is good and the vac transducer is new. how does the system work please explain from start to finish on every thing EGR related! please! and one more thing,....do you know what gear ratio exists in the front and rear diff. i'm putting 33" tires on but want to retain my MPG's haha if thats possible. thanx for the help hope to hear back soon! take care.
-Matt-

Answer
The first two codes are a problem with the distributor, the RPM pickup coil is located inside and not replaceable, this also causes loss of the ignition signal, I can't give you a complete discription of how an egr system works at this time, however if you'd like you can give me an email address in a follow up I can send a document that will explain everything with an illustration of all the part involved.  As far as I know the gear ratios of the front and rear differential are the same and are interchangeable, they used either a 4:11 or 4:56 ratio.