groundrushpull

I have a 1998 Subaru outback wagon auto trans. 2.5l engine. 145,000 miles. The wheels were rotated by tire shop I have been using for 20 years about 5,000 miles ago. I was traveling this weekend and noticed what I thought was low tire pressure about 130 miles into the trip so I pulled off and checked and the right rear was missing 2 lug nuts/bolts sheered off and the third basically fell off when I checked with my hand. I had not noticed anything unusual until this trip with most of my driving being 20-30 miles per drive or less. What would cause the bolts to sheer that way....I understand loose bolts could sheer with wobble but what caused the nuts to loosen? I have heard that with an all wheel drive there could be a problem with the transmission when it is malfunctioning and keeps trying to transfer power to a side or a wheel thinking it is slipping such as in snow etc. which could cause the sheering? I did have another instance of the front right nuts coming loose a couple years ago but did not have anything sheer off as this was noticeable everytime I would take my foot off the gas and found this right away. I have had a problem with the transmission where sometimes it does not want to shift into drive right away and I have to wait or rev engine for it to catch and this happens with no real pattern hot, cold weather or sitting overnight or just driven. Thanks for any advice you could give me to solve the problem John