alright i know im new here, but id definetly appreciate your guys help with this.......

heres the deal its a 2003 1500 5.7L Quad Cab 4x4.......anyway it has that *** that selects either 2wd. 4wd hi, 4 wd low....anyway this recently happened the trunk makes a absoultely horrible noise in 2wd and it jolts and jumps about, but as soon as i put it in 4wdhi it runs fine..now obviously its a pain running in 4wd cause it make tight manuevering alot harder...but here the kicker in 4wd only the rear wheels will spin...as i checked in the rain and it is definetly sending the power to th rear wheels......so it makes horrible noise and jolts about in 2wd but when u stick it in 4wd it stops but only the rear wheels are being driven....at first we though maybe a halfshaft or something but i dont think thats it..so i figured id ask the experts on a dodge forum.....can u guys help me...i plan on diy for this unless its gonna involve a press to push out bearings or something.....and ideas