Please Help

[bballmac42]

Now please dont get mad. I know this is a 67-72 forum, and i have a 68 c-10, but i have a question about my newer truck and there are a lot more people in this forum then the other one. I have a problem. I have a 1989 silverado single cab shortbox. I have a 1989 chevy shortbox. I am getting a vibration in the front end when i hit 45mph. It continues to vibrate even when i speed up , but it gets less noticeable because it vibrates faster. When i let off the gass at say 55 it stops, but when i step on the gas it starts up again. I crawled under the truck and looked at the four wheel drive drive axles, and the passenger side seems to be loose (able to move in and out a little bit and side to side) The driver side doesnt do it near as much. Anyone know what my problem is?