So it really is only a jeep XJ body ,not a real jeepsure loved by all sure.jeep has 2003 avalanche LS5.3 ENGINE AND 4L60E TRANS WITH A 6" Lift (ouch) on 37" tires with 1979 Ford 3/4 ton rear ends.with NP231 transfer, np207 gone now with new NP231 transfer case from the '91 YJ ?, NO, from a CHEVYI bet now it is NP231C box, the C means Chevy box.with 2H, 4H, N, and 4L,, this is part time 4wd, not full and is not for the 4wheeling on pavement. only offroad or on ICE.ICE means, just that or snow or snow over ice. (sorry captain Obvious, just left the building,now)end ICE answersbegin, 6" lift extreme problemsthe real problem is the LIFT that high.3 "easy4 hard6 "extreme lifts.IT fails in 2wd mode too, that is because U-Joints have limits. (all do) and the lift span chart below.(angle) post.nothing else needs to known, but U-JOINT ANGLES are all WRONG.SOLVED,. the 6" lift done wrong. YJ is a very short Wrangler and very short rear prop shaft.that means Prop(rear) drive shaft angles are dead wrong. (solved the reason on 1 fact)DOne with no SYE kit. "slip Yoke Eliminator"'https://www.amazon.com/Heavy-Eliminator-52231-18676-60-Driveshaft/dp/B00FDWL7PMdone i bet with no shims for rear axle LEAF spring perches , to get angle right(they sell shim kits)the answer You need a double cardan or CV (same piece different name) at the TC case, this will require a new yolk. You will also need to shim the pinion angle to get the driveshaft and pinion within 2 degrees of each other.