The vibration w/ tcase in 4wd and hubs unlocked is odd...
You said you rebuilt the front axle. Did you reinstall the outer c clips on the ends of the outer axles (birfs)? Did you stake the pinion nut on the front diff? Have you confirmed that there is no radial play in both the pinion flange and the front tcase flange? When you had the front axle apart, did you check the condition of the brass bushings in the front hubs?
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln