Shifted to 4th and shakes ensued
 

I was driving my truck today and as I was getting onto a freeway I shifted from 3rd to 4th. When I went into 4th the truck started shaking very roughly. This continued to 5th gear. When I moved back to 3rd the shakes went away. Any idea what could be causing this?

1989 pickup 4x4. Front wheels were locked at the time. 22re.