If it's a bad enough wobble that you have to come to a near stop to get it to end, search the forum for death wobble. If it's merely a vibration you can easily drive through, it's likely a tire balance issue or possibly a bad ujoint.
I too have a wobble/vibration on my 87 wagoneer, and it was the same issue with my 92 cherokee. Mine only does it between 45 and 55 mph...otherwise its completely smooth. Any thoughts?