i have never seen this setup . but just looking at your pic i can see the flex point all the way on the rear end . all the pressure on the lever pivot point = flex back there with no hold down anywere .
make a tab to pickup 1 or 2 of the 6 bolt pattern bolts and come over and weld to the back end of that bracket . that should firm it up a lot .
but on a side note : stock np205 single levers shift like crap . they shift so much better when you twin stick them . and this lets you run 2wd low on a stock np205 . if you get the upgraded shift rail you then can do front wheel drive only option .
but the 2wd low only extra gain of a twin stick is so nice for backing up trailers .
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info you gave with early gearratio's of nv4500 . but your shifter style shows later trans unless you swapped on later top cover .
also i hate to tell you this but your old combo 465/205/3.08/33 = 2195 rpm @ 70 mph . your new combo 4500/205/4.10/33 = 2133 prm @ 70 mph .
but you will get a animal of a truck from the gearing in the axles with 4.10 over 3.08 .
play with this cool web site
http://www.grimmjeeper.com/gears.html