Engine Light, Vibration, and a thump or bump at 65.

I have a 1996 Chevy Astro AWD Cargo Van with the v6 and an automatic transmission. It has 154,000 miles on it. I just bought it about 3 months ago. I am planning a brake job next month. I just put 4 new tires on. I found out it has 8 inch custom rims. There was a horrible noise when I turn the wheel too far, mostly in reverse. That seems to be gone since I put on a slightly narrower tire. The ABS light has been on since I bought it and I am getting maintainence caught up as soon as I can. (Brakes right after the 1st) It also needs the 150,000 mile cooling fluid change.

Now I have a new problem and I am not sure what is wrong. When I drive it I get a vibration that rattles all the doors and windows. No noises what so ever. I think the engine seems too pause or hesitate with the vibration, but that may be my imagination.

This started on the freeway at 65mph. It felt like a thump on the gas pedal or perhaps like what if feels like when coming down from a hitting a big rock minus the other parts of hitting a rock. Then a few miles down the road it did it again. After that the vibration started at over 50mph. I limped the van home.

The vibration lasts for maybe a minute. Seems like forever but I'm pretty sure its just a few seconds. I don't hear or feel a shft. Not grinding noises, nothing. Now I can't go 10 blocks without that vibration and I occasionally get the thump too. I would really like some idea of what the problem could be before I go take this into the dealer for repair and fluid change.

Also the service engine soon light is coming on and going away, coming on and staying lit for couple start ups then going away again.

Is this all one problem or perhaps 2 or 3 and what could they be? HELP!