It's been my experience that it's hard to beat the white-lab-coat guys who and gals who designed the vehicle. If you encounter ride issues that were not present previously but show up as the mileage wears on, it's best (IMHO) to track what went wrong with the system as designed and work from there.
Does this happen solely on concrete freeways? Is it at a certain speed range? Does the vibration comes up through the steering column or does the whole cab shake?
Just my 2 cents here, but there's alot of things I would check to ensure that the vibration wasn't coming from worn or damaged parts before I started dropping $$ on upgrades. Here's a bit of a checklist, from most obvious to least ... all this you can DIY or have done for $100 or less, this complete inspection is about an hour, all labour. If you choose to DIY please follow safe jacking procedures and use jackstands always.
1) Check tie rod ends, ball joints etc. for any play. Those wide ol' 20's tend to be hard on front end components, as they generally weigh more than standard wheels and tires. 'Specially a 4x4. Sandbagging a truck tends to numb out wear-related suspension issues rather than fixing the root problem.
2) Even if your tires look great, they may have rolled or broken a belt (hit any tasty potholes lately?). The 20's with the thin sidewall tend to soak up alot less impact before the rim gets bent... worth your while you have her lifted and check the tires and rims. Spin each one, putting your hand across the tread surface and feel for irregularities as it revolves..even a slight hump magnifies wildly when the wheel is spinning at 1000's of RPMS. If you find one that looks or feels wonky, have it balanced to check. Also check inside the rims from underneath, if you see evidence of loose or missing stick-on wheel balancing weights, you may have lost one when it came unglued. Re-balance any wheel you feel suspect, or do all 4 to be extra sure.
3) While you're under there, check out the universal joints (front and back) for excessive play, they can throw weird shimmies when on their way out. This may sound kooky, but look at the driveshaft as you have someone spin a wheel, I had a spooky vibration solved by almost accidentally noticing scuff marks on a driveshaft- turns out friends' son had gone 4x4'ing with his buddies and driven over some gnarly stuff, smacking the driveshaft off a boulder and bending it ever so slightly, resulting in a hard-to-diagnose, boneshaker-style vibration.
Hope this helps.