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The most likely problem is (are) the radius arm bushings.  The one on the right
side is the one that usually fails since it is next to the muffler and gets
beaten to hell and back there on the rought side of the road anyway.  Another
poster suggested drilling the rivits in the brackets that fasten the brackets to
the frame.  Use bolts to reattach.  You will need some patience and a damn good
drill with a carbide bit to do this.  Around that vintage,  Ford even had a TSB
somewhre along there as a fix for a pop noise in the frame under some
conditions.  If you go that route, make damn sure you get the proper bolt
hardward to hold it together.  Those bolts have to be tight and good enough to
stay that way.

If you do have a pop in the frame, you may want to inspect the area around the
steering gearbox for cracks.  Some did that.  There was a reinforcing kit
installed to repair this.  Sometimes, the gearbox bolts were simply loose and
all was well after tightening.

Another prime area for poping and crackling on them is the front crossmember
rivits.  If there is any indication this is the proble, the only solution is
drilling out the rivits and replacing them with bolts.  Again, these are special
bolts that could be had in a kit at one time.  If you buy  other than the kit,
make sure they are SAE grade 8 nuts, bolts and washers.  Again, they will need
to be tight. Check the torque specs for the bolts and put it to it.

Every one I have ever replaced the bushings on, I found it just as easy to put
it up on the stands and drag out the air tools.  Disconnect the shock and lower
the axle with a jack.  Loosen the bolt that attaches the radius arm to the axle
so it can move.  IT is also recommended to remove the upper spring clip to the
frame so the spring doesn't interfer with pulling the axle forward.  Remove the
large nut at the rear of the radius arm and pull the axle forward out of the
frame bracket.  A come-a-long may be handy for this if you have the truck so it
can't move and something to tie onto.  A long pry bar can also be used to move
the axle forward if you have a helper.  Install new bushings and reverse the
disassembly to put it back together.  Tighten the nut on the rear of the radius
arm befor the one on the front to make sure it is not in a bind.  You will know
when they are tight enough when your 1/2" impact can't go any more.  The washer
on the rear of the arm will bottom against eh shoulder of the arm.  I recommend
you spend a couple of extra bucks and install the urathane bushings.  It helps
the overall handling of the truck and correct a couple of nasty traits like
pulling ir darting when you apply the brakes or hit a rut in the road surface.

For you drive line thump, you need to pull the drive shaft and lube the splines
with a good EP  grease.  That may only silence it for a while before it returns.
Then you have to decide whether you want to repeat the lube process every so
often or replace the front yoke.  Yoke replacement is usually a permanent fix if
it is installed with a good EP grease.  Some yokes come with a zerk fitting to
facilitate periodic lube but, you have to be careful not to overdo it with an
automatic trans.  Keep in mind the U joint has to be disassembled to replace the
yoke.  This would be a good time to replace U joints if they have some miles on
them.  If you have never done a U joint replacement, you would probably be ahead
of the game to pay the local driveline shop to do it.  An unbalanced drive shaft
can loosen your fillings.  Note: this problem  does not occur since the
driveline is not loaded when stopped unless you are a screwball riding the
clutch.

All this sounds like a lot.  Once you get started with the right tools, you
should be able to knock it out in an afternoon if you pace yourself on the adult
refreshments.

Good luck

Lugnut