I replaced the detent spring and flushed it several times, but the selector is still seized up real bad. There’s hardly any rust on the truck but any aluminum looks calcified beyond belief. It’s the NP 231 (I believe it’s aluminum). Flaky and scales all over. A lot of corrosion. Best I can tell is the selector is stuck internally, as I soaked it in PB blaster and smacked the linkage back and forth a million times with a hammer. Wire brushed around the selector bolt that goes inside. Hard as **** to move it still. Got it where the front was disengaged and drove it home. With it being in 2wd, would the front driveshaft / u joints still cause the vibration. Like OP said, there’s nothing in my steering wheel. No shakes, shimmy or pulling.
Sorry to hijack, hopefully this gleans some new info for OP too.