I'm surprised it let's you get to 60 before a vibe. Get a jack under the tranny and support it. Then pull the skid off and check everything. Who knows what the PO did down there. Make sure to pull off the trans mount and inspect it. You often can't tell with it still attached to the trans. Replace if needed. Rubber mounts best control vibrations. Find the transfer leak. It could be the output seal from the drive shaft flailing from lack of skid support. If anything you might have the axle pinion a degree or two high. Won't know until you get everything around the skidplate corrected.
There is also a fair chance that the engine mounts are wiped out from having everything hanging behind them.