OK, I'll throw in my $.02 here, with the caveat that I'm only attempting to resolve any doubts you have about what you already know. It seems you've systematically and logically isolated the problem area. So with that said, here goes:
the driveshafts are both brand new double-cardan 1350s done at same time as swap. i should have posted this the first time - but when the front is removed, there is aboslutely NO vibes - so yes we have the issue somewhat isolated.
I agree that you've isolated where area the problem lies.
the old setup that had some vibes was with double-cardan 1310s, again they were done brand new at the time of my first axle swap, and also removing the front would make it drive with no vibes then too. i had the front re-balanced TWICE to make sure that wasn't the problem, then gave up.
OK, you had this problem with your former axle setup, and with the front ds removed, didn't have the problem. That seems to confirm your first point above. Also, assuming you had a reputable shop do your driveshaft work, I wouldn't think ds balance is the problem. And I believe you most likely did source a reputable shop do this work, after reading all your posts on your build. So again, my guess is that this is not the problem area.
if anything, i would have expected my vibes to go away due to the new axles being hi-pinion and therefore having less angle on the front shaft.
I agree, but since you did have a vibration problem with your old setup, and you've now corrected (or changed) your driveline angels, it now appears that driveline angles weren't the real problem. Of course, you wouldn't know that until you got to this point, with new axle assemblies and corrected (lessened) angles.
since nearly everything (both driveshafts and both axles, gears, etc) has changed twice now except the t-case - i'm leaning towards there being a t-case front output issue. it is a used 242HD with like ~50k miles on it. other than output bearing, what inside the case could affect the front only? can the output shaft be bent, how likely would that be really? are there certain internals that i could have put back together wrong when i did the SYE?
Well, I haven't broken into a tc, so I can't directly give any input from that perspective. However, based on the above history and the things you've already done to troubleshoot this problem, I'd say that there was something wrong with that tc as a component in your old setup, and the problem was not related to driveline angles. Was there something wrong before you swapped the SYE? I think that's the probably source of the problem, and that's where I'd start checking. Did you put it back together incorrectly when you did the SYE swap? Well, I suppose that's possible too, but that would only have exacerbated an existing problem. That's where I'd begin - pull the tc, dissassemble and inspect the internals, starting with the front output shaft.
and arlo is correct - the one suggestion was to put in a double-double-cardan in the front. but i feel like this would be just fixing a symptom and not the root cause.
Definitely agree with this - although, Arlo's suggestion may work, I think you really want to figure out what the root problem is, not cover it up with a "work around".