jeff,
i assume you mean the grand cherokee m/c with booster won't fit in the position of the original dodge m/c, yes? it is not impossible to use a booster with the original m/c, however. many of the 1.5 ton and larger trucks were fitted with a remote vacuum booster and the single-chamber m/c (mine has this). the vacuum booster is located to the rear of the cab, inside the frame rails, behind the fuel tank. although not [likely] offered in the 1/2, 3/4 or 1-ton trucks, i wouldn't be surprised if it was possible to retrofit a "bigger" truck vacuum booster to one.
wally