bell housing bolts tight? flex plate bolts tight? you sure it's not the trans mount? motor mounts?
FWIW I had a bone stock 1999 XJ Classic with the 4.0, AW4, 231, as stated....no mods..bone stock. under hard acceleration from a dead stop, no wheel spin, it had this drive line vibration, only under HARD acceleration. that turned in to drive line vibes when I lifted it. eventually went SYE and that cured everything.
'94 ZJ Limited 4.0, BB, 31's, the rust free California transplant.