98 Rodeo front vibration

I have a 98 Rodeo that has major vibration starting around 60 mph. The car only has about 49000 miles on it and has been well taken care of my by mother-in-law. I had all 4 tires replaced and spin balanced, but it had no effect on the vibration. I had a friend put it up on a lift one day and discovered that the front wheels were in 4wd, without the transaxle engaged. It was like I had a car with manual hub locks and I forgot to unlock the hubs. After pushing the 4wd button a few times, it disengaged. But shortly thereafter, it seemed to go back into 4wd mode. I notice that the gas mileage on the highway is really bad. My father-in-law said that once he took it on a long trip and managed to get about 28 mpg. I got about 18.

I notice that when I am coming to the end of a braking maneuver, the car lurches forward. I don't know what is going on, can anyone help?

New shocks could help the vibs.

28mpg is unheard of, 22 is the best I've done.

Check out my FAQ link below for some help on MPG and a couple of other things.
-Ryan

I was thinking more along the lines that the vibration was coming from the transfer case since the hubs were locked in. I guess I am looking for some trouble shooting tips on the vacuum assisted shifter used on the front axle to put the car into 4wd.

Shocks may be a small problem. When I did manage to unlock to hubs, the car rode very smooth at high speed. I think that the vacuum system may be slipping the car into and out of locked and unlocked hubs. Is this a problem anyone else has experienced?