Driveline shudder, caused by a worn U-joint.

Isn't it common fo the lock-up unit on these to be a failure point?
Wouldn't that shudder at very low speeds as it tries to lock up prematurely?

Richard -- I think I've got your answer.  I chased the exact same
symptom for a year in my '97 318 @ 103K because of its variability:
some days it was there, some days it wasn't.  Eventually I was led to
discover the fragile nature of the magnum cylinder head -- cracked
valve seats.  Not enough to see in a vacuum flutter or compression
test, and not big enough to push coolant out or suck it in, but enough
to get CO in the antifreeze.  The ever-so-soft cylinder just fires with
a little less gusto under high vacuum conditions -- your no-throttle
take-off.  As soon as you crack the throttle open, the higher load
completely masks the symptom.

Run a test for CO in your antifreeze, and/or a leakdown test if these
conditions make sense to you and your truck, and let us know.

FYI:  think about where you put your money....I was in a bind on a
Sunday afternoon, and we magnafluxed a dozen "core" units -- all
cracked on the valve seat.  I settled with an awful pair of new Asian
castings for cheap money to get the truck running, but one failed
before I was "ready" to sell it and move on...so personally I rolled
the dice with an ebay reman'd OEM unit.  For my money, however, the
bare pair from Hughes plus a local valve refacing would be the way to
go...it's priced right.

Jon

Thanks all, I will start with the u-joints since its easiest and cheepest and
work through the other solutions, thanks for the input. Richard