sp1249

I have a '92 Cherokee that has a random vibration that comes and goes as it pleases. The maximum duration of the vibration is about 5 seconds and then it goes away. It seems like hitting bumps or just going up a hill will set it off, but not everytime either. It is most prominent when I'm pulling a trailer, so, being under a load seems to be a factor.
I have removed the driveshaft and checked both u-joints and they are fine. I replaced the original transmission mount that was collapsed and thought I had it nailed, but that didn't change anything either.
Anybody have any ideas?