1996 E-350 vibration at hwy speeds

Our '96 E-350 had all ball joints replaced 3 months ago; since then, it has developed a bad vibration when the gas pedal is released at highway speeds (55-65 mph). This happened every time, until I noticed the tranny fluid was low; I added a quart, and that helped a little, but it still occurs a bit. Any ideas??

u-joints?
lost a weight on one of the wheels?
bushings in the suspension?

Kim MG,
Thanks for the ideas. If it only happens when we stop accelerating at certain speeds, might that rule out the tire balance idea, and maybe the suspension too? Also, it seems to be happening in the rear wheels (the u-joint might be the issue). Could it have been damaged by low tranny fluid, and if so, what's involved in the repair?

Thanks for your help!!

A U-joint wouldn't go out because of a tranny problem usually. Another thing that you might check, and this is rare but I'll throw it out there. On coast down you use the backside of your ring gear teeth. When I was at the dealership, I saw a few dana 80s chip just the backside, usually from backing trailers into something causing a big bind-up in the driveline. Like I said, it's rare but if you exhaust all other options, it's something to look at.

Thanks for your help--
It turns out that the problem was the u-joints, and it was caused by a cracked flange/yoke in the transmission; that messed up the u-joints. Seems to be running fine now--thanks everyone for the help...

Brooke Hopkins