since a rod shouldn't be overloaded can one be "underloaded" if i throw a 3/4 oz. swimbait off a rod that is made for 1-5 oz lures will that affect the action of my lure??

No, it won't effect the action of your lures, it will effect how efficiently you can cast them, though. It depends on the rod, too. Sometimes the rod ratings are pretty arbitrary. As a point of reference the Cumara 7-11 MHXFast is rated at 1.5 ounce lures, yet will throw a 6" HUdd ROF5 with no issue at all. That bait is rated at 2 ounces. The G Loomis SWBR956 is rated at a 3-8 ounce lure, however it excels on lures in the 6-8 ounce range. In no cases, though, are you going to be in a situation that it's going to dramatically effect your lure action, the only time it might is if you're ripping baits.