over night i thought of a way to possibly see if a panhard bar, or track locator would help solve your problem. if there is enough unobstructed area around your lower, parallel, 4 link arms, you may be able to fab up a temporary diagonal link such as used on a ladder bar setup. it would attach to the drivers side at the front and the passenger side rear mount. [or the opposite if need be] the bar would be 1" or so in diameter with rod ends at each end. the brackets made from 1 1/2" or so 1/4 wall tube pieces with the appropriate spacers required for the ends used. as i used 5/8" heims, i used 1 1/2x3x1/4 wall tube for the brackets. or, you could just buy a diagonal link kit from [?] to save time provided you can find one that will work with the length you need between your lower brackets. pop in a couple of longer lower bolts and see if that does anything. if the symptoms improve or are eliminated, leave it installed or then fab the appropriate brackets needed for a panhard bar. just something to think about. it may work, may not. it could be something else altogether. wheelbase is well within workable tolerance in my opinion.