Vibration After Lift, BUT in 4Wheel Drive Only?

I hope this isn't too stupid of a question. I spent an hour trying to search for another thread on this, but couldn't seem to find the right combo of words. I just put an RC 4.5" lift on my '91, along with a Tom Woods SYE and CV shaft, it has a 242 transfer case. 2Wheel drive is smooth! No vibes of any kind at any speed. We got some snow recently, tossed it into part time 4wheel drive and hit the highway, I am getting vibes, mainly on decelaration. Also a bit under load, like going up a hill. Hoping this is something simple as my "budget build" is turning into my money pit! Sure would appreciate your guys' input! Thanks for looking.

Check the ujoints in the front shaft would be we're I start

did you do their replacement shaft, or a hack and tap? if you did the full shaft, id remove the drive shaft... and drive it in part time. dumb, i know. but if it still vibes, your install would be to blame.

take a good second to inspect the drive shaft and Ujoints...

if that looks good, work your way down the axles.

Maybe I should just have the front driveshaft rebuilt and balanced. Old girl has a touch under 250k and I'm sure it's the original driveshaft. I sprang for the whole replacement output from Tom Woods, didn't like the h&t idea. I get zero vibes in the rear. Now again, forgive my ignorance, but my head is telling me front AXLE u-joints would be felt in both 2 & 4 wheel drive, but something with the front driveshaft only in 4 wheel drive? Thanks for schooling me fellas.

No your front shaft spins the whole time unless you have locking hubs
So all axle and driveshafts spin