Im a Jeep mechanic and I own a 08 myself.....6-8 inches is alot man if its indeed that much....Im gonna guess mine moves a few maybe 3 inches...climb uder it and look at the tranny mount...could be loose or broken
as for reverse you have found the weak link in this tranny....Reverse is weak...not the gears or synchros themselves but the shift rail...The lil plastic peices that slide like to break off...theres a bullitin up to 06 on the 6 speeds...
I have prob done the shift shaft job 8-10 times in the last few years....
The general symptm is that it pops out of Reverse when you release the clutch
I havent seen it on the 7s or 8s yet but knowing Chrysler I doubt they have actually fixed it...Ill check the bulletins tomarrow and see if its on the 7 8 models but I cant remember seeing it...Gary