About the gearbox and diffs, there is no getting rid of the sound completely, some of it will be there, it all depends on how much is it, if its too much, have it rebuilt and the grinding will probably go away if you are lucky, mostly it does not help, the only solution is to get a NOS one or modify some other one into it. You've got a diesel funker in it, might as well bolt another diff, since you've got it looking like a Barbie Jeep!