1999 Explorer 4X4 AWD staying in 4WD

My Ford Explorer has also proven to be an Explosion, Bought used from
family member who had no problems, 2 weeks after buying just dyed on
interstate going 60 mph. Found out had been special ordered (1 of four in
US) and motor had been recalled and never taken in for repairs, of course
well past recall notice time so we had to find a used motor and replace
EXPENSIVE!!!, now immediately after engine change. While driving it keeps
going into low and high gear (Automatic with button on dash to turn on and
off) lights will come and flash while driving. Have taken to several
mechanics (1st one guy who replaced motor- he has no idea) Ford dealership
said could be when engine was replaced guy got wires crossed . One guy says
we could have a sensor out on one wheel. What do you think this could be,
these three guys say this is their first experience and have no real idea
of what is wrong. This just caused us to have to have a drive shaft
rebuilt (the one that runs this particular 4 wheel drive mechanism.
(ordered not installed yet).Please give any advice ideas!