OK, so I wasn't going to hold my breathe waiting for a response here since I was serious about solving this dilemma pretty quickly, and I DID solve it. Last night, finally... Hopefully, this will help others
The 98-00 ENVOY OEM bulb is NOT a D2R nor a D2S as some have suggested before. I went on ahead and purchased a USED original equipment Envoy bulb on eBay the other day just to see it in person, and to make some precise measurements and a formal comparison.
Please see my 18 captioned pictures here: http://www.ojbox.com/blazer/ENVYvsD2S.htm
...which thoroughly show the details of this problem and how I fixed it on my Blazer with Envoy light fixtures. These pictures I took are not perfect, but they do get the point across.
Turns out that the Envoy bulb has the EXACT same distance from the flange/bulb-seat to capsule/filament as the 9006 bulb. This might make sense as the original non-xenon GMC and Chevy trucks of the same year range all used 9006/9005, and maybe some goofball at GM thought they should all be the same, or that this was the 'ideal' focal point in a perfect world or something like that. But really, my simple logic is just telling me that they just wanted to build a proprietary bulb so that they could charge an arm and a leg for the replacements. Car companies can't get away with this type of behavior as much anymore though, since many Americans are loosing their loyalty with the whole 'Made In the USA' slogan and putting their money towards foreign made cars where those manufacturers don't pull these selfish stunts to such extremes (I mean $500 for a light bulb is a bit ridiculous).
So in my case, I opted to modify the light fixture to be able to correctly house an unmodified D2S bulb. But the other option is that you can get a 9006 HID re-based bulb (verifying the re-based distance is correct before actual install), and you must also modify the mounting tabs of the 9006 bulb base enough to make it fit into the factory Envoy fixture and then build a retainer (kinda like I did for mine in the pictures) to hold the bulbs perfectly in the center of the bulb socket, and firmly into position. I do have a set of 9006 rebased HID bulbs here (see the pictures in the above link) which DO have the correct focal point as a normal 9006 bulb (I know, since I rebased the original rebase job since the original rebase job from the original bulb peddler was crap, but my re-base job I did on it was perfect). But the main problem was, that these 9006 capsules I got are not even true Philips or Osram and the support braces for the capsules are not even very heavy duty (very light weight) and you can flex them to some degree and the bulb just doesn't feel very secure), so I opted NOT, to use those for this. And since I do have plenty of Philips 85122 OEM D2S capsules laying around, it made more sense to me, to mod the fixture to just take a D2S out of the box. But the easier option for many who lack the tools to effectively mod the fixture and get the dimensions just right, then I would recommend installing 9006 re-bases and just Dremmel the mounting tabs just enough so that they will drop into the Envoy sockets, and the 9006 depth will be the same as the Envoy HID bulb. If you choose to use the unmodified D2S bulb, then you need to do something to make it sit 5/32" (of an inch) deeper into the fixture. This is the magic number that I paid $200 to be able to measure.
Does anybody want to buy this OEM Envoy bulb/igniter, (complete with ballast-to-igniter harness wire) from me now? I will probably just put it back up on eBay now at this point unless someone here wants it, then maybe we can make a deal.
Hopefully, this material will help someone out there