Tom in terms of fuel economy you should be seeing low to mid 20s in normal use, a bit better if you stroke it.:augie
Your problem sounds like either a bush/panhard rod thing (easy to see if you just manhandle anything in sight under the back end) or something to do with more drive to one side than the other....the LSD uses a clutch pack system, so it could be that one side had seized maybe.....and it might be that the new oil (which never gets changed and causes lots of issues) will sort things in due course.
Mind you, the engine has plenty of torque; are we talking wet or dry road when this happens, and how much wellie exactly?
And does it do it in 4WD (only try it in the very wet - its NOT intended for dry conditions as you'll get transmission windup).