860 jumps out of 2nd gear

Since it is jumping out of 2nd it is likely to be a worn detent plunger. On the left side of the transmission housing are 3 plugs lined up vertically. They have a slot for a large screwdriver. Remove them and underneath you will find a spring and plunger. The end of the plunger is half of a ball and it seats into grooves in the shift rail. If the ball is worn it won't properly lock the shift rail in place. The 1st/2nd plunger is the middle one. If it looks worn try swapping it out with one of the other plungers. If this fixes it you are looking at maybe $10 for a new plunger.

If it doesn't fix it you are looking at tearing into the transmission. While you are in there you might as well replace all the seals and any bearings that are worn. If you do all of the labor you can probably do it for $500 or less in parts.

My 861 was jumping out of 3rd and it was due to a worn sliding coupling. I got a good used one for $50 but I spent a total of about $500 replacing seals and bearings.

You will need a genuine Ford service manual to do this job.