knocking/shuddering on initial engine load

i have a '95 jeep YJ and i've been getting a series of knocks (more at shudder) under an initial load. seems dependent on the amount of torque, as i can ease into 1st w/o hearing the noise. conventional wisdom has been that this is a universal joint issue, however, inspection does not reveal excessive play or looseness. could it still be the u-joint?

i first noticed the noise after having my tranny rebuilt and cat. converter replaced, but it sounds like it's coming from the rear, w/in the drive line. any thoughts?

thx,
J