Strange knocking clunking front end problem.

1995 Ford E250 4.9L L6, cargo van 176k miles.

Alright the problem is as follows... When driving at slow normal speeds
going straight i get this horrible repeating knocking/clunking sound
seemingly coming from below the center rear of the cockpit floor, though
perhaps more towards the front right wheelwell.. It is difficult to
pinpoint exactly where the sound is coming from. The sound is similar to
the semi-random but cyclical thud, thud, thud sound you get when putting a
pair of shoes in the drier. The sound only happens at 'neighborhood speed'
going straight, if i get over approx 35 mph it goes away. Also if I'm
making a sharp turn in either direction it goes away. I've tested this out
in a parking lot- when driving circles in both directions there is no
knocking. The sound also *immediately* stops anytime I apply the brakes
even lightly. And of course, there is no sound when parked or idling. The
smoothness/roughness of the road being driven seems to have no effect. I
can be driving on a perfectly smooth blacktop parking lot and still get the
sound.

There is also a noticeable 'CLUNK' whenever I first engage the drivetrain.
ie from parked to 'D' or from 'D' to 'R' .. or parked to 'R' .. etc..
though this may be normal for the van.

This may be entirely unrelated, but the van also makes a deep
vibration/rumbling on the highway from 55-70mph... which is another problem
I'm working on fixing. Tires were already balanced twice. I already
replaced the u-joints and plan to check out the front wheel bearings and
replace the transmission extension housing seal tomorrow as it was leaking.
If these don't help I may take the driveshaft to be balanced.

Tomorrow I'll be jacking up the front end to swap out the tranny filter,
fluid, and extension housing seal + bushing. I'd really appreciate any
pointers for items to inspect while I'm under there or any further tests I
can do to try and narrow down the problem spot... keep in mind im only a
beginner/novice here.

much thanks as always
Jay