could be your tires. i run 38" toyo radials on my crew, no problems what-so-ever. when i throw my bias swamps on, i get d.wobble at 30mph. if i stay at 30, it gets knarly & have to almost stop. if i speed up a bit or slow down a bit, no problems-just can't drive at a steady 30. my steering gear is all new, kingpins done couple yrs ago, caster is bang on, dual s.stabilizers. only does it with the swamps. took them into the tire shop, he flipped them on the rims & added some weights...
their a little better, it takes longer for it to DW at a steady 30 now, & does not get out of control when it does. one day i mite ditch all the lead & give Dyna Beads a try....