At around 350,000 miles I started experiencing all of the same symptoms. Put in a new nv5600, clutch, throwout,pilot, clutch pivot, clutch hydraulics... . tried all the different fluids in the transmission. . overfilled, etc. In other words, I tried everything and anything to make that darn thing go into gear and shift smoothly.
The best I had obtained was to just hold gentle pressure on the shifter, while shifting, and eventually it would go into gear.
Now..... I told you that... to tell you this.
The latest issue of TDR arrived and as I was reading the article on the harmonic damper I got to wondering. The article made a passing comment about difficulty shifting. I immediately went and examined mine. (yes, I had been neglecting that like most people. ) Drat! Mine looked just fine.
But. . I reasoned that at my high mileage, the truck being almost 10 years old and it would be something "shiny" on the truck I ordered a Fluidampr just to see. (it's "only" a couple a bunch of C-notes, right?)
The Fludampr arrived yesterday, I installed it immediately ( about 30 minutes to remove belt, unbolt 4 bolts, remove old damper and install new one)
Result? I put almost 200 miles on the truck last night driving around I was having so much fun. It was like a new truck. I can shift with two fingers!!! And... a vibration that has bugged me for many thousands of miles is gone. Whodathunk?
A post-mortem on the original damper showed that, even though it look fine, the elastomer (rubber) was intact and in place, it had stopped being elastic. It was almost brittle. As such, it would the damper wasn't a damper anymore. It was just a small flywheel.