Join Date: Sep 2006
Location: Vancouver USA
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The main difference between the 18 & the 20 is that the 18 has an offset rear drive while the 20 has a centered rear drive; this effects where the pumpkin sits on the axle, either off set to the passenger side or centered. In reality the late model 18 and the 20 cases are interchangeable but there is no way to add the overdrive to the 20. So as a 73 Commando has a centered rear end and if you install the 18 your dive line would not line up which could cause you problems.
1978 CJ5, AMC 401, CJ T18, Scout D300, 2 1/2" Lift, Locked Front/Back