on my 90 that i had, the truck would do something similar now that i think about it, it turned out to be the rear end had chipped a ring tooth, and then the whole rear end went BOOM a few months later...

__________________
99 extended cab, 4.0, auto, 33s, 4x4 lifted with some stuff done

98 2wd 2.5, 5 speed, dropped with some stuff done