Grinding and shaking only below 15 mph
 

This has been driving me nuts for months! Let me start by saying I'm no newb to fixing odd noises or my Xj but anytime I cruise around slowly below 15mph or so my Xj grinds and shakes like ones business. I've gone thru everything including: all u-joint, Rear axle swap, regear front end w/ new carrier, bearings, and spiders, new drive shafts, mostly new steering joints and steering box brace. All of this is stuff I've done since this problem started. About a month before it started I did a SYE conversion and everything seems fine and engages correctly, I've checked both t-case fluid and tranny fluid,both are clean and metal free. Only thing I notice is my speedo needle like to jump around a bit (+/- 3-4mph). Could it possible be a bad t-case output bearing? I noticed when I installed it that the rear tail housing bearing had some surface rust on it but not a ton to be concerned about. Is there something I may have overlooked on the SYE install? I thought maybe a tranny issue but wouldn't I notice other issues in different gears and higher speeds. Bc everything shifts fine. Any thoughts? Bc I don't mind doing a t-case tear down again or a tranny swap, but I don't like doing explorative surgery on a transmission that may be fine. Please throw in some input.