Ford F150 - Bad shaking when starting to accelerate

I have an 02 f150 fx4 that i got stuck in the mud pretty ba. Upon getting pulled out, i washed off all the mud and had to have the tires rebalanced. Now i'm noticing a bad shaking in the rear when i accelerate. ANy ideas how what could be wrong?

I am having a similar problem with my 99 f150, although i didn't get it stuck first, it just started happening. When I accelerate I can feel a vibration in the rear end. I've replaced U joints but that didn't fix it. Still not sure what the problem is...

My 99 XLT 4WD 5.4 is doing the same thing (see post "Rumble/vibration after warming up" under the Brake/suspension/exhaust/undercarriage" forum) Like the post says, mine only does it once I've driven it 8-10 miles. There was once when it started vibrating while driving down the highway, but that has never happened again. It always rumbles when I accelerate from a dead stop or when I accelerate up hill. It also rumbles when I'm going really slow with the brake partially on - like when I'm pulling into my garage. So far no one has replied with any ideas. I haven't had time to take it in and get it seriously looked at, but I did have my mechanic ride with me when it was doing it and he was baffled.

sounds like the driveshaft is outta balance