Newbie driver with questions?

Hello all, i have been around the net trying to find some answers but no go. I've just started hauling fruit for a family farm and they have me in a 2003 Freightliner M2 with the MBE900 and an eaton 6spd. My question is about the rear differential. Its a Meritor RS-21-230 2 speed and i think i may have an issue but being that i am brand new to all this id like to learn as much as possible before i take this up with our equipment manager and ruffle feathers that don't need any ruffling as he purchased this truck because of my complaints with the #### box i started in.(I don't want to complain if i don't need to) Anyway the rear differential has about 5 inches of free play and by that i mean if i spin the drive shaft it turns almost 3/4 of the way before it catches something. Ive looked through the fill plug on the rear diff and the ring and pinon turn with the drive shaft but i think its the two speed part that's allowing the excessive? travel. Also when the diff is in low range the light blinks and only stops once in high range. and when the transmission is put in gear and you start to take off it bangs pretty hard and after watching its coming from that rear diff. The drive shaft spins and slams into what ever is catching and than bounces back. Any info or help would be greatly appreciated.