I just installed some and had the same issue. First, the metal cups in the rear bushings aren't supposed to sit flush with the differential facing side of the urethane like they do with the OEM rubber, they are inset. If they're right out against the end of the urethane then it will be very hard to push them into position because they fit so tight. Their position doesn't matter as far as installing the diff goes because the front bolts will hold it in place and tightening the nuts on the rear studs will pull the cups into position so just have them about halfway into the rear bushings and get the front installed. Second, even with the cups in the proper position it will be a tight fit to get bolts into the front support bar. I tilted the subframe up on it's back end, got one of the long front bolts started and then used some pushing on the bar to get the other long front bolt started. After that tightening the bolts up some allowed the two short front bolts to slip in easy.
