Shift Kit

I don't think both would be a good idea. The Jet power shift looks like it would work just fine by it's self, so I would suggest that first and because it would be easy to put in/take out if you don't care for it.

Dont waste your money on any of those chips, you will not see a difference. If you were to get any I would get the Hypertech because you can change gear ratios and tire sizes with it. A custom tune will be more beneficial, however on a stock engine you will not see much of an improvement.

I know this is an old post, but... I don't have first hand experience on the chips and Jet brand products, but I agree with George and recommend the Hypertech products, although, on the '99 Blazer I have on my profile I DID see a big improvement using the Hypertech Programmer III. Check out the Dyno charts for your Blazer at hypertech.com. But to answer your question, anything you plug into your OBD II slot re-flashes the computers memory, i'm guessing the Jet Module would just over-ride any setting the performance chip has. Bottom line, go with the Hypertech versus Jet, you have more performance options.