I have around 53k in my 1999 AWD Mountaineer. For some odd reason, when I drive at speeds around 35-45 mph, my vehicle "shakes"... it doesn't seem like a vibration ... it kinda jerks, as if it's having a hard time changing gears ... at the same time, i feel it on my seat. I'm not sure if this is an alignment issue or not -- at first, I thought it was a tranny issue. I changed the tranny oil, put new filter -- but that didn't help. After that, I drained the transfer case fluid and filled new oil in the tranfer case -- that didn't help. Could this be as simple as an out-of-balance tire? Or do you think this is more serious -- as in a damaged tranny, damaged driveline componets? I took it in to a shop, and the guy said it could possibly be my U-joint. Any help/insights/ideas/suggestions are greatly appreciated. Please help!!!