yes, you can absolutely order a 3.4L and install it.
that auto won't have overdrive, so be prepared for that.
do you have the VIN? the guys can look up the build sheet.
I do believe they are made from a rare rust-able tin foil. 'tis the bane of MJ owners everywhere.
are you re-torquing the nuts after a drive around the block?
how hard do you need to pull on it to get it to engage? I could see myself maybe fabbing a bracket that attaches to the center console in my truck. you could tie in to the 4wd shifter bracket for strength.
the difference in MPG is essentially whatever it takes to spin the driveshaft around. very tiny compared to pushing a 2 ton brick through the air.