NP205 Experts: Knowledge Wanted!

[joeydurango]

Asked over on the general board 2 months ago but never got this figured out.

My 72 K10 (350/350) has suddenly begun spinning the front driveshaft intermittently when in 2HI. I'll be coasting along down a hill, all's normal, hit a few small bumps, and the vibration appears to cause the case to s l o w l y begin engaging the front shaft. I can hear it, and feel it, catching just the tiniest bit, then increasing engagement.

The front axle and hubs are all good, I rebuilt them a couple years back and checked them again recently. Everything else (shift linkage, all connections, u-joints, driveshaft slip joint) looks/feels completely normal and is tight and solid externally, so I've got some sort of problem inside the dang case.

What part in the NP205 t-case is responsible for keeping the t-case locked in 2HI? Is there a detent spring or something that could have broken, allowing some selection mechanism to "float" between 2HI and 4HI even when the shift lever is solidly in 2HI?

TIA!