Have your front rims checked to see if they are true, this shimy is common with a wheel balance or tracking issue. I get this big time on my LWB GQ, if i have lumps of mud in the rims after being off road but i ussaly find I can drive over or under it until I get home to clean up. Is your truck ...

If you have a 24v truck and you tow frequently, your local auto sparky should be able to fix you up with a little relay unit that converts your trailer wiring plug to 12v This is just a box of small relays that are swtched on the 24v side but the output side of the relay's to the trailer is supplied ...