Figured that after your clarification of terms. Like I said, the NP-231 and NP-241 are basically similar. There are severel model variants, mostly with internal gear ratios.

The NP-241/2 is known in the Jeep world as Command Trac and is a full time 4x4. The NP-231 is the most common.

Again, when you lock it into 4x4, you are fully engaging the front drive. If you get vibrations then, the factors are driveline angle, driveline balance, worn bearings, u-joints, front tire balance and alignment, loose or worn spring hangers. If the runout you measure is within factory specs, I would eliminate all other factors before focusing on the driveline.

I overlooked the question of where (how) are you feeling the vibrations? You said the when. If you feel it in the steering wheel, I would focus on the steering components such as tire balance, ball joints, and steering linkage. If you feel it in the console (at the shift lever) I would focus on mounting issues.

I have experienced vibrations due to driveline balance, worn u-joints, t-case mounting, and tires. Tires can be fine at low speed but cause vibration at high speed due to internal issues where the steel-belt is off center.