Does disabling the MDS via tuning or putting the RAM into 5th or 6th gear manually, have an affect on engine wear?
No, the 8 MDS lifters disconnect the top and bottom parts so the valves stop opening, and the PCM cuts fuel to those cylinders. The pistons are still moving up and down, around the crank, lifters are still following the cam. I suppose the valves, pushrods, and rockers may get a little less theoretical wear, but not anything you'd ever know about. This is something Chrysler specifically tested for 13 years ago when they implemented it.