I think I'd start with Mark's advice, considering he worked at Ford as an engineer for many years. Stop driving the truck before you truly wreck that transmission and get it fixed.

I since replace the oil in the differential didn't find any broken pieces. Also bought a second-hand transmission and put it on the truck is no longer locking up the rear end but now seems to be staying in 2nd gear and will not allow me to shift into 3rd gear or high gear. Wondering if I should try to replace solenoids in first transmission or not

If that second hand (junkyard?) transmission had any sort of a money back guarantee, return it. Try another junked one if you can't afford to repair or replace the original.

I began having the same issue
 

Did you find the solution to your problem? I began having the same problem on my drive home last night and your description of the issue is the best. Driving 50+ in a 60. I believe I was driving straight when it first happened. Tach stayed or may have risen. Speed dropped. No change to trans temp 170. No loss of battery, oil, or fuel, and no increase in engine temperature. Seemed to occur 40+ mph. I did hear my tires churp once. No smells of any fluid. The only thing I don't really know is if the truck pulled to the right as this happened on a two lane road with cars coming at me and behind me, therefore I instinctively pulled to my right to avoid hitting someone. But honestly, I don't think it pulled. Wife just passed away so I am trying to avoid taking it to a shop and mis diagnosing it. Today I will regreese the drive shaft and check the differential

Both rear tires locked up?

Tires locking up p while driving
 

I'm having same issue. Extremely scary when it happens. I've changed trans solenoid, pcm, and inspected every wire I could see. After I changed the computer it drove fine all day. The next morning on my way to the swap meet I made it 3 miles from my house and it started all over again. I even replaced my cluster because it was randomly shutting off. Any ideas. I love this truck but am at the end of my rope

Which trans solenoid? There are seven of them.

If you're just looking at the wires you are wasting your time. You MUST check the wires electrically. Can you see through the insulation to see a broken wire? I can't. And what if the wiring issue is somewhere that you can't see by looking at the truck? Get a multimeter and check each wire for continuity from the PCM to the trans, and for short to power and short to ground.

And even before that, check for codes. That may tell you exactly where to look fo the problem. Get Forscan at www.forscan.org. Their page shows how to select an OBDII adapter to plug into the OBDII port. I bought mine from Amazon for less than $30. Once you have there you can read all of the codes. Otherwise, you could take it to a good independent shop of a Ford dealer to read the codes. The places that read codes free CANNOT read Ford transmission codes.