It could possibly be a bent or out of balance rod/shaft in the driveline. Anywhere between the engine through the driveshafts possibly. Just shooting in the dark. Definitely not gonna be fun. Good luck. Let us know when you figure it out. Hopefully someone with lots of knowledge will chime in.
1998 TJ, 2.5" OME springs, OME N66 & N67 Sport Nitro Shocks, 33x12.5 Goodyear MTR's, Metalcloak front trackbar, JKS rear trackbar, Metalcloak aluminum front arched fenders, Metalcloak aluminum front bumper, thedirtworx tire carrier, Warn xd9000i w/ synthetic line and TRE Safety Thimble II, Monstalined interior