Join Date: Apr 2006
Location: Crescent Springs, KY
I was wondering the same thing a while ago. What I was missing is that the engine has a mount on either side that acts as a pivot. The next mounting point of the driveline is the transmission and transfer case crossmember/skid plate. When you install those spacers, the transmission and transfer case come down and the engine pivots up at the front a bit, and down at the rear. This changes the angle of the rear driveshaft and reduces stress on the single universal joint and therefore reduces driveline vibration.
Many folks who don't want to lower their transmission/transfer case crossmember/skid (and thereby loose clearance) install a slip yoke eliminator (SYE) and constant velocity (CV) equipped driveshaft. The SYE shortens the length of the transfer case assembly (allowing for a longer driveshaft) with the end result being to relax those driveline angles similar to the lowering kit. The CV driveshaft has 2 universal joints to distribute the angles further, again reducing stress on the joints and shaft and therefore reducing driveline vibration. I also understand that adjustable control arms and/or longer than stock control arms can also figure into the equation, but I'm now at the limit of my understanding, so perhaps more knowledgeable folks will chime in.
'99 TJ Sport, 4.0 I6, 5 speed manual transmission, Hi Lift w/offroad kit, Viair 450C compressor, Garmin GPS, Skid Row Engine/Transmission, steering box & radiator skid plates, Kilby gas tank plate, AtoZ Fab full length rocker guards, Performance Accessories 1" Body Lift, OME 2.5" HD springs & shocks, JKS trackbars front & rear, BFG AT 33x12.5r15, EBC Yellow pads and Centric rotors