Have a bad vibe at 50 that just get worst from there. Have gone thru rear end, new u-joints, balnced drive shft, frnt whl bearings , rear tranny mount, and even tried a different harmonic balancer. Had it in to tranny/ driveline shop who went over all my work, serviced and pressure tested the tranny and said they couldn't find it either. You can feel it in the seat but if you lean towards the windshield you can hear it's up front???? It has a 4100 250 efi, rear drive. It runs great gets 24mpg! Any ideas????

what about the braking system?

vibrations usualy come from steering wheel troubles. or should be some alignment problems.

happened to me once. but i got some tire pressure issues in my case.

if you're not too sure it maybe best to check ti up on your dealer before diving in.