5EAT Shudder?

Background:

2008 LGT, second owner - bought it used in early 2011 w/~48K miles. Currently at about 90K. All maintenance (60K, 90K, 105K, oil changes) and upgrades have been done exclusively by IAG.

Daily driver, driven spirited but not beaten (never launched, etc.). 5EAT Transmission.

Upgrades:

Perrin FMIC

VF-52 turbo

Walbro FP

GS EBC

Invidia Catted DP

Prosport UEL Headers/UP

Tuned by Jorge (~265HP/280TQ)

At approx 85K miles, started having temp issues related to HG. Car never overheated, only elevated somewhat on the gauge. Replaced shortblock with a whole host of additional related parts (oil pan/pump/supply lines, etc.), Did 90K and 105K services at that time. Just finished the 5K oil change (back to full syn) as part of the short block replacement. Driven easy and then even not quite spirited for the break-in period.

Issue:

This week, car makes fairly decent shudder (felt in the steering wheel) as I pull in to a parking spot. RPMs drop slightly too. As I shift to R to adjust parking position, shudder goes away while in N, then reappears in R.

When idling in gear with brake applied, symptoms are fairly repeatable in R, not so much in D and barely/occasionally in P/N (moreso in P).

At driving speeds (normal/higher RPMs), nothing... car shifts fine, nothing noticeable. After driving for 15-20 minutes, symptoms seem to reduce (sometimes). I also notice a 'thunk' when the engine fires/turns over (after cranking).

IAG has spent the better part of the past two days diagnosing the issue. Here's what I know so far - it's not the mounts, the axles, suspension. I believe they've even disconnected the driveshaft with no conclusive results.

Maybe the transmission? I've heard/read of the 5EATs failing through higher gears, but from everything I've read about these symptoms, it points to axles...

Thoughts?

If it does end up being the transmission, what options would anyone in the MD area recommend? (IAG doesn't service/repair ATs.)

Thanks,