RJF,
I just read your thread. As others have suggested, try the 5" Bosal tips first. We both have 05's and that was the first year that GM put the LS 2 engine in them. The stock exhaust in them has a lot more rumble then the older model trucks. I think that is the main reason why folks with older trucks are putting new mufflers on. They want to get the tunes from their exhausts that we may already have. That and the fact that they might need new mufflers. As I've mentioned in some earlier threads, I had the OEM tips with the diffuser holes on the inside taken off a couple of months ago and put on the 5" Bosals. There was a noticeable improvement in the rumble and I find that I don't need to replace the muffler AND as an extra which no one has mentioned the 5" tips have given my R more power because the opening of the tips is larger and there is no back pressure on the exiting exhaust. At least I think that's the reason. Now, I have to apply more pressure to the brake pedal when approaching red lights because the engine just wants to run. Try the tips first on your 05. I really think that may be all you need and when your muffler wears out you're going to have to put a new one on anyway, so you can explore the muffler situation then. I hope my advice is not too late.
