you'd be better off to buy a bag of playground sand and throw a few handfulls of it in there. There are several products that resemble sand (equal, counteract, magnum) on the market, but they are like $10-$20 per bag. If you do this, I want to hear your first hand experience on whether it really works or not. I have no first hand knowledge of this working for anyone, although I have heard plenty of people claim these products work.
The biggest problem with these sand type products is the manufacturer recommends using very little lube when mounting.
having the tire trued is a better idea, although you will lose tread depth.
BEFORE YOU DO ANY OF THAT, CHECK FOR MISMOUNT!!
To make sure the tires are mounted properly: Most tires have a guide rib molded in near the bead area. Take a small metal ruler and measure from the edge of the flange to the guide rib. I would look for less than 1/32nd difference between the largest measurement and the smallest measurement. Measure in about 6 places (more is better, but on a 15" rim, 6 is probably plenty). If you have more than 1/32, you need to break the bead, re-lube the wheel and tire, then re-seat the beads.
Did you clean the wheel's bead seating surface before mounting the new tires? If there is rust or just a nasty buildup of crap on the bead seating surface, it can inhibit the tire's ability to get a good seat on the bead.
Did you check the bead seating surface on the tire? This can also have an effect on how well the bead seats.
Take it from me, Many RVs (that are using truck tires and trucks for that matter) end up with vibration problems that can be traced directly to a mismounted tire.
but they are still really bad. When I get over 35mph it shakes the truck in a scary kind of way. Got ideas?
Looks good with the big meats 
