So this has been going on for a while now.
when shifting from first to second there is a rattling sound from under the center console, and again from second to third but not as noticeable.
mostly it happens when starting up a hill and is not present going down hill.
2007 Rubicon/ RK 3.5" of actual lift/Rancho 7000 shocks/Teraflex monster TB/Synergy highsteer and tie rod/Hankook MT 315-75-16 on Level 8 ZX's/MC front SB links/Rancho geo brackets/Fox ATS stabilizer/Adams front DS=Frankinlift from hell.
Could be tranny mount if it sounds like your 4wd lever is buckling in the floor boards, is there vibration? There comin to fail, put a bar on your tranny and see if she wiggles. If it is it can take out your pas. side engine mount. go poly!
i recently had the same problem with a rattle at certian rpms and shifting, mine wound up being the cat was bad. the core was busted and rattling. easy to fix either cut behind cat about 2-3 inchs. grab long metal bar or long screwdriver and break up what is left of internals of cat and remove all that is loose. get a exhaust band clamp and reattach the cut exahust with exhaust band clamp. hope this helps...