vibration when hubs unlocked

[704x4]

I have been running my 70 k20 (79 fame) with no front drive shaft for the last two years to do away with the full time 4x4. I fainally got around to putting mile marker hubs and conversion kit in my 203 this winter. At first it worked fine, but the other day when i left home it started vibrating so bad that I couldn't drive over 30mph. So i pulled over checked everything visually, and it looked ok so i pulled back out on the road and it drove fine. Later that night it start vibrating again, but only over 60mph with the hubs unlocked. When i lock the hubs it doesn't vibrate anymore. I thought it was the front drive shaft, but im confused why it doesn't vibrate when the hubs are locked and the tcase is in 2wd. Any suggestions on where to start would be appreciated. Thank you!