Great question — answering as the UphillTech team:
I wouldn't worry about the dropped packets. Doing the math, you're losing less than .001% of your total packets. As far as the load balancing goes, you probably need to (re)configure the load distribution algorithm in use on your switch (since it's the switch to server traffic that's not balancing). Cisco has some pretty good documentation on that here but it's too much to copy into this answer. Also note that the distribution algorithm assigns traffic streams to a physical interface which means that if there is only one stream it will only use one physical interface.
Full answer by David King: https://unix.stackexchange.com/a/251189 (CC BY-SA 3.0).
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