rightsmanaged, I have the setup you have, P, R, N, D, L.
60-70 mph in D like to sit at 2100-2300 RPM although it goes up and down by itself to meet power needs. never tried over about 45 or so in "L" but it was still pulling at that point. maybe yours is stuck in "L" or, as Gramps says, its in failsafe mode.
I would hit the dealer.
2000 Cherokee Sport 4.0L Selec Trac - mine - for occasional wheeling
2007 Patriot 2.4L FD I 4x4 - wifes - doomed to rarely leave the street