Comparability between 3 different vehicles

My question is, I have 2 semi running vehicles, a
1996 Jeep Grand Cherokee and a 2000 Mercury
Grand Marquis LS, according to some of my
research some parts can work on both vehicles. So
another part of my question is can some of the
parts on a 1996 Dodge Dynasty work for the
Mercury? I know that some of the Dynasty's parts
can work on the Jeep, so I am wondering if some
of the Dynasty's parts will work for the Mercury.  
My daughter owns the Dynasty and is having a junk
vehicle buyer coming to pick it up, but if I can save
some money and help her out also by scavenging
parts it would be great. My Mercury is having fuel
pressure problems and the Jeep has a blown head
gasket, the Jeep is my boyfriend's project vehicle
or I would scavenge from it. Out of the 3 the best
running (when it wants to) is the Mercury and the
Jeep is second. The Dynasty was kinda crap when
my daughter bought it.