Who is an NP205 expert?

[SilverMiner]

I'm stumped and I hope somebody here can help. I just finished rebuilding the NP205 in my 1980 K20 Suburban attached to a TH400. The rebuild went extremely well with the help of Off Road Design, some knowledgeable friends and of course YouTube videos. The reason for the rebuild was to replace a stripped 32-spline input shaft. A twin-stick conversion was done a few years back when I first installed the TH400.

The problem today is once everything was back together (on the bench), placing the Rear Axle rail into HIGH locks everything up. The input shaft will not turn and feels bound tight. Both shift rails move smoothly, and if the rear is placed in LOW everything turns fine. With the rear axle in LOW, the front can be engaged in LOW and everything is still fine. If the rear is in NEUTRAL, the front can be placed in either HIGH or LOW and everything turns. Upon sliding the rear into HIGH the input shaft is bound.

HOWEVER - messing around with it last night, I discovered I could make everything work perfectly by removing the poppet, spring, and ball from the rear axle shift rail. WHY? I don't understand how simply adding the slight compression of the poppet spring could bind up the input shaft via the shift rail at all.

I welcome any and all theories you may have. I'm confused enough at this point I may just have to blame it on the Russians.