A rough idle on the 91 vette could be from a variety of reasons. Bad, worn out, or loose spark plugs. Worn out or damaged spark plug wires could be the cause. It could also be the optispark (Distributor for these cars). I have also seen fuel injectors create a rough running condition on these cars as well. Start simple, and don't go to deep in the beginning look at the surface issues and look deeper as you rule out the most obvious culprits. There are many things on these cars that will cause a rough idle but it is better to rule out the cheap and easy to fix causes first. Check the plugs, do a compression test, test the wires, coil, and injectors and what not before you start throwing parts at it. The compression test is a good thing only because these cars run really hot, and some don't engage the cooling fans until 240+ degrees, so it is possible to see a crack in the head, head gasket leak, ect on these especially if the cooling system has not had a good maintenance record.

You won't find an Optispark distributor on a 1991 Corvette. The 91 vette has the L98 engine, which uses a more traditional distributor. 1992 was the first year of the Generation 2 LT1 engine that uses the Optispark. It is true that many issues can cause rough idle. Rough idle can often be caused by a vacuum leak. Check for leaks, including a ruptured bladder in the EGR valve, which will act as a vacuum leak. So, that is something else to consider.

What kind of Jeep?

Generally a auto tranny with overdrive will have 4 if it's an older model, probably only 3

If it's a newer model it will have a 5 speed; the 2013 have 6 speed; with the talk of 8 speed comming in 2014 .