Mostly upper intake plenum seals on the 3.8 V6.  There are updated
seals and sealed bolts to cure this.  The intake manifold to head
gaskets can leak vacuum (among other things) as well, but they are a
less likely cause of P0171 and P0174 codes, and more time consuming to
replace than upper plenum seals.

The seals shrink esp. when cold, and cause a cold start vacuum leak
sometimes accompanied by a rough idle.

Toyota MDT in MO

Mostly upper intake plenum gaskets leaking on the 3.8 V6.  There are
updated bolt seals and gaskets available.  You can also replace the
intake to head gaskets as they can leak vacuum, but to a lesser extent;
the job is more difficult as well.  The seals shrink quite a bit when
cold and cause excessive vacuum leaks on cold startups, resulting in a
rough idle or sometimes just the P0171 / P0174 codes.

Toyota MDT in MO