My 2003 Dodge Ram 1500, 5.7L shakes at 45mph, and gets worse with increasing the speed. I have to drop it below 40 to get it to stop. I noticed when my 4WD is locked in, it does not shake the slightest bit, no matter the speed. Any ideas? Recently had wheel bearings replaced, and all new tires that are balanced.

 

I had the same thing happen with an '02 1500 that I had before. It ended up being the CV axles and front differential stub shafts. The CV axles are just held onto the stub shafts by a chincey little clip that sits in a detent on the stub shaft ... the CV axle clicks onto it and stays there.... supposedly. My CV axles both worked their way out of being fully seated on the stub shafts and were flopping around when I drove.... like you said it kind of went away when I put it in 4wd. So maybe take a look at the CV's and see if they are fully seated on the stubs. If this is the issue and the stubs aren't destroyed.... you might be able to replace the clips and fully seat the CV's and be done with it.... no major surgery.