Shaking Caravan

When my 92 Dodge Caravan (3.3L V6)reaches 45 miles per hour, or is climbing a hill it starts shaking. If I try to go any faster the shaking gets worse and feels like a tire has blown. I was told that a broken steel belt inside a tire could cause it. I relaced all 4 tires with little improvement.

The shaking stops when i take my foot off of the gas. I can coast down hills at speeds above 45 with no shaking.

the only event that i remember happening around the time the shaking started was a flat I changed roadside just prior to start of the shaking.

anyone have any ideas?

Have you motor and transmission mounts checked.

What caused the flat? If you ran over something large it may have bent a wheel.