Chevrolet Repair: 95 Chev Suburban 7.4 with rough idle, high RPMS in park, hard shift to drive, chevy dealer, thing thing

Question
We have just replaced the plugs, wires, coil, distributor, cap and rotor, and the truck is running terribly now ( was running quite well before ) It has a high idle when in park, and clunks into drive. It chugs, runs rough, and is slow to accelerate, if it chooses to at all. We have had to drive down the highway at only 60kms......There is a ton of black being shot out upon start up, and we also just had an entire new exhaust system put in, going from single to dual. Not one of the mechanics we have taken it to (including the Chevy dealer) themselves can figure it out without charging a ridiculous amount to do a tear down. Could this be the EGR valve or perhaps a vacume leak? If so, how easily can these be replaced/repaired or cleaned?

Answer
HEy,

 One thing thing that easily gets missed on those is the reluctor magnet on the distributor.  They get cracks in them and spin back a bit affecting internal timing of the pickup in the distributor.  
 A stuck EGR would cause a rough and black smoke idle but would not adversely affect the engine management under load because they are supposed to be open at that point.  
 What kind of prices are they talking about?  Most diagnosis can be done (as long as its not intermittent) in under 2 hours.  I charge a minimum of 1 hour at 90 dollars to look at drivability.  I can most always identify the problem within an hour or 1.5 maximum.

Wayne