[DanEC]

Will it do this sitting still in your driveway? If so I would Jack it up and replicate the blocking, shut it off, climb underneath and pull the rod off the transmission lever and see if you can manually move the transmission lever into 3rd. If it goes into 3rd OK then it’s probably a shifter issue, but if not then probably an internal transmission issue. The interlock rods and shift detents are another potential area but if it was rebuilt and shifted fine for 4K miles I would think they are OK.