My '99 Tahoe has missed a bit a low idle but ran great at highway speeds until the other day. I was driving up a mountain and dropped out of overdrive and the truck began shaking violently. I thought it was a flat but ruled that out quickly and decided it was the engine or transmission. I slowed to pull over and it stopped shaking. The check engine light came on. I got out to inspect. Nothing obvious. Drove it at regular speed to the dealer where they suspected cadalytic converter. The error code was a misfire on cylinder 4. They ruled out the cc and then said my injectors were dirty. They flushed the injectors and test drove and had another engine light with shaking. This time cylinder 3 was misfiring. They wanted to replace all 8 injectors for $1300. I started to do it but then hesitated. Why would 2 injectors fail at the same time? Not likely I imagine. Do you know what might cause this? What else should I look at first? If it is injectors, does anyone know a do-it yourself guide to replacing injectors on a Tahoe? Thanks in advance.