Jumping in here, and not meaning to be an @$$#0!3. Your combination of tire size & gear ratio should be fine with a 3.6L. Be sure you have the tire diameter programmed correctly, along with the gear ratio when you use the Procal. Measure the tire diameter with a tape measure at street pressures from the ground to the top of the rear tire(s). Don't program in the rated size from the sidewall.
'09 JKRU. 4" Teraflex long arm, Teraflex R44 housing, Teraflex / Fox Resis, Poly Performance skids, Nemesis Billy Rockers, Nemesis flares, 37" ATZ P3s on 17" 5x5.5" Trailready HDs, Spyntec hubs, Teraflex BBK, Mopar BBK master/booster, Superior front & Alloy USA rear axle shafts, CTMs, Reids, Synergy drag link & tie rod, WTOR hydro-boosted steering, 5.38s, TF Dual Rate front bar, TF bumpers, Warn Powerplant, Tom Woods shafts, AA xfer case cable, Kilby canister relo, RH4x4 cage.