'97 Explorer front end "clunk" while accelerating

I just got my '97 AWD V8 back from Ford after they replaced the rear
main seal (ouch). Maybe in my head, but something wasn't right with it
since I got it back.

Today, I was pulling away from a traffic light at about 10 m.p.h, and
stepped on it. As it was accelerating, I heard two loud "clunks" from
the front end. It is usually two clunks, but sometimes only one. It is
driving fine otherwise. If I am at speed, and step on it, it will
downshift, but no clunk. Seems to only be from low speed. It sounds
like front diff, maybe transfer case. Doesn't really sound like
tranny, but who knows - they pulled it to get to the rear seal, so
they coulda screwed it up. It's almost the same sound as when the
universal joint went on the front drive shaft, but that clunch was
constant, and the car was barely driveable.

I did notice *another* leak coming from the pinion seal on the front
diff, but I doubt it is leaking enough to drain it. Probably gonna
need all of the seals in the front axle replaced anyway, so if it is
the diff, maybe only a little more painful than what I need anyway. I
did the rear pinion seal myself, but knowing what I now know about the
bearing pre-load, I think I'll leave this one to the pros.

Anybody have any other ideas from my lame attempt at a description of
the problem? I hate taking it to the dealer when I don't have a clue
what it is, and I hate taking it to a local shop, 'cause they'll
rebuilt the whole darn car on my dime, regardless of the problem.

-Joe