My xj did this when I had mis matched, un evenly worn tires. I have a 3 in lift and running 31s. I tried putting the most squared off ones up front, and still, awful death wobble between 46-48 mph. Started off as a sometimes thing and eventually it happened every time. Funny, but the more muddy the tires and rims were, the less wobble I got. Finally got a set of evenly worn tires and had an alignment done, and of course, balanced the tires as best as humanly possible... No more shaking to the point of a full on wobble, but it still has a small 'shimmy' that doesn't bother me at all.
Ps, When I took the old tires off, I hosed down my rims and got several pounds of mud off that was caked on, too.