What engine would be a good replacement in a 2003 Ford Explorer?

I have a '03 Exploder sitting in my driveway, currently only driving me from
crazy to insane in less than 6 seconds every time i look at it.  Everything on it
is fine, except for the engine, which i'm pretty sure has a blown head/head
gasket.  Now, i have a selection of used explorers and rangers on my local
craigslist postings, and i'm tempted to pick up a used husk of a ford with a
working motor to pull and pop in Dora here.  My question, after months and
months of research, can i pull any 4.0l V6 from any ranger/explorer and pop
it in, or do i need to try and stay in my "generation" of explorer?  Are there
any other fords i may want to look at for an engine swap(aerostars come to
mind)?  Also, would there be anything i need to look for if i go for a different
engine, like the 4.6l V8?  Are there any other fords out there with an engine
that will a.) fit in the engine compartment and b.) bolt up to the tranny?  The
only info i've found is about direct engine swaps, nothing much about
upgrades or variation.  My concerns are this:  IF i go for another 4.0l(reman'd
or used, either way), it seems like it will likely blow again since a good chunk
of the CL listings for explorers in my area all seem to have bad
engines/heads/gaskets.  I haven't really had another ford besides my dad's
old '94 ranger, which SEEMS to have the same engine, but never ever blew
out on me.  So, given this, i feel i am left with one of three choices: 1.) risk
the motor blowing up again and drop the 1500 on a reman'd 4.0l, drop the
same 1500 on a reman'd 4.6l which does NOT seem to have a rep of
blowing up, or find something local and used for cheap that i can drop
in(literally anything to get it moving again). My ideal option would be to take
one of the rangers/explorers for cheap of local CL and drop it in, but again, i
come back to my initial concern:  will any ranger/explorer engine work, or do
i need to keep the parts vehicle in the same gen?