1969 Ford F-250 Slipping into Drive and reverse

Good day,

My old truck has been sitting for around one year without being used. I start it to move it around our small yard to cut the grass etc. When I started it recently, after sitting for a few months, I let it warm up, then put it into drive. The truck went nowhere for a moment, shook, then slowly started to move. The same happens in reverse. It will shake while backing up. I thought it might be like my old 79 Ironhead, and maybe the clutches etc where stuck together due to not moving. I tried going forward and reverse in the driveway to see if that loosen anything, but it doesn't. I noticed that if I turn it off, then start it warm, it works ok a few times, then starts again. The truck is not insured for the road, so I can't, and don't want to at this point, drive it on the road to see what happens. I would like to put it back on the road, but not sure what to do at this point. BTW the truck worked fine the last time I parked it before leaving it for the year.