A picture would help.
I was going to comment earlier that a vibration in the dash but not the steering wheel might be rear drive shaft issue.
The front drive shaft issue would most likely also be felt through the steering column if it was severe enough to be felt through the dash. It may be possible if your drive shaft is spinning with the front wheels and the shaft at the piece of drive train, the previous owner installed, (possibly an SYE) and transfer case is loose, it may just transfer vibration directly up through the frame/ body to the dash.
Again would be easier with a picture to see what this piece is.
I believe front driveshaft may run full time in a TJ unless you have a lockout hub conversion. I can't remember if my 97 had one.
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