Hello. I have a 1988 Ranger with about 162k. It's been a perfect vehicle for 10 years. A few months ago it started idling rough, like a surging sound. At high speed it ran great. I thought it might be the fuel injectors so I tried a bottle of cleaner. There wasn't any change.

Since I'm not a mechanical genius I took the truck to the local mechanic. He said he needed to do a tune up. Shortly after that he said he needed to change out the valve that feeds fuel from the tanks to the engine.

Next thing I know I get a call that the swapped the valve and now the engine is seized. I went there myself to look at it. It's truly frozen, in both directions. The mechanic said there was no noise, no nothing. It just died. A co-worker said they had the same thing happen to their '89. Is this a common problem in late 80's Rangers?

Thanks in advance for any insight on this strange situation.

That sounds pretty sketchy. I would think you would hear bad things before the engine seized. Since the last thing they did was fuel related I would pull all the spark plugs and see if the engine turns. (Think hydrolock). The statement about this happening to their 89 seems like a classic defense move.
It's always a tough call when someone else is working on your stuff and there is a calamity. Who is responsible and who should pay for the disaster.
If this happened to me I would want to disassemble the engine far enough to determine what happened.

As a side note I don't believe that engines from any year commonly seize without any warning.