Transmission will not shift out of first gear - 1989 Ford?

I have a 1989 Ford F-150 with an automatic transmission, and lately have had some issues with it.

I had to drive 25 miles, and around the 20th mile I had to take an exit to get where I needed to go. The truck ran fine up until I turned onto the exit. When I turned on the exit, the transmission was "stuck" in 1st gear, and would only go 20-25 miles per hour. I ended up having to drive back home (20-25 miles) with it stuck in first gear, taking taking extreme care to be gentle on a back road.

The night before, the truck ran perfectly fine - nothing odd. It just all of a sudden failed. I bought the truck around five months ago, and has ran fine since with no issues.

We were able to get to a Walmart to buy transmission fluid. The truck is known to have a transmission fluid leak, but I keep a very close eye on the transmission fluids (check it with my oil - about once a week) to ensure everything is as it should be. As soon as I poured transmission fluid in, it immediately leaked back out onto the ground.

When I got back home, I had the truck towed to a garage for them to look at it. I originally thought it was a seal had broken, but the mechanic said that all the seals looked perfectly fine. He said he'd have to do more thorough tests (for $90) to find out exactly what is wrong with it.

Is there any ideas as to what is wrong with it? How much could it possibly cost to fix? Mechanic said could be pump or governor. Also, transmission made a loud thump when placed into drive.

Added (1). Transmission fluid is full, according to mechanic. As mentioned, entire truck (transmission and everything) ran fine the night before and that morning until we got around 20 miles away.

Points

Check the kick down cable. Both three and four speed transmissions have them. Check it at the top and bottom also if a three speed it has a governor if that's stuck it won't shift. The four speed a common problem is with the cable the crommet on top of the cable breaks check it on top of the engine. Find local shop at ATRA.COM for more help if needed