Issue with transmission fuse in 2001 F-550 w/ 7.3 diesel.

My uncle has had this truck since 2002, bought it brand new after it had sat on the lot for awhile. At this point it has in the neighborhood of 400,000 miles. No longer his main truck, but needs it when he has to haul sand.

Every time he puts it in reverse, its blows the fuse to the transmission. When the fuse trips, the truck can't go faster than 7 miles an hour. Which is idling in 1st gear.

Sometimes just moving past reverse on the way to drive from park trips it.

Sometimes the fuse also randomly trips while rolling down the road. When it does this he has to throw it into neutral, or the transmission basically becomes a giant brake. Has to reach down and switch it out, even when rolling down the interstate at 75.

His mechanic can't figure out why the fuse keeps blowing. He has taken it to a few others as well, but they can't find the issue either.

Any ideas? Has anyone experienced anything similar?