One small update - the mechanic it's with has had the relief spring out, and said it was completely free, so it looks like that shouldn't be a problem.
If that's free, is there any other way the pressure could go this high?
It's looking more and more like the sender.
1991 Grand Wagoneer - Hunter Green
1998 Cherokee (XJ) 4.0
1997 Grand Cherokee (ZJ) 4.0
1974 Ford F100 390