I would think veeeeeery carefully about replacing a spring with a rod. A rod is cheaper, so why didn't Mopar use it? Because the spring cushions the shift & cushions the shock load on trans parts. The Transgo shift kit came with a stronger L/R servo spring, no rod.
The rod goes under the accumulator piston, not the L/R servo. The L/R servo is modified by some shift kits to add a shim to essentially make the servo solid for a faster apply/release of the rear band.