My shifter used to rattle on the highway too. It was due to a driveshaft that was a little out of balance. The handle rattled a little from looseness but new wooden grips from Gunslinger tightened that up.
Some of those aftermarket shifters, like the Competition Plus, need the special adapter bushings to take up the bigger shifter lever hole when installing the stock e-body linkage. Get them from Brewers if you need them.
Rob