shortbus

I have an 85 Brat I bought a few months ago. 4sp EA81 with 149,000 miles on it. The problem I'm having is a REALLY bad vibration sometimes while on the gas at highway speeds (45+ MPH). When I let off the gas or push in the clutch it goes away but as soon as you get back on the gas it starts again. I'm 99.9% sure its coming from the front and I think its from the passenger side. From searching around on here I figure its probably axle related. My main question is how can I tell? I've jacked it up and there seems to be quite a bit of freeplay in the axles when I turn them. Is there anything else I can check? I found an 83 wagon in pretty good shape in a junkyard I just want to be sure the axles are the problem before I go through the trouble of trying to pull them out of the wagon. Thanks for any help.