Its not a powerstroke but ford none the less.....

I have an 84 F250 which currently only starts with starter fluid. The shop replaced the fuel pump and filter and I drove it home no problems, started right up. 3 hours later it wouldn't start. Original problem was it didn't want to start in the morning, they said glow system checked out ok. did the pump and filter. The starter cranks but engine doesn't attempt to turn. I have to take it to a dealer now because they can't diagnose it. I don't know if this has anything to do with this but I am only running on my rear tank, I can't select my front (bad selector valve I'm guessing) but both tanks are full.
They think it could be low compression, If so is this serious? Just got the truck a month ago and it runs great when its started and warm. No apparent leaks underneath. I just want to know what I'm looking at when I take it to the dealer...don't want them to stick it to me to bad.
Thanks

Don't go to a dealer unless it is a last resort. They always try to screw you. If you do don't let them convince you of anything else being wrong. You'll get ripped off. Low compression could be a serious problem. I would get a 2nd opinion at another independent shop. Try thedieselstop.com. I think that's what it is. They are more about Ford diesels. What kind of temps do you have?