I have a 2006 ranger recently bought it had been park for a bit,but I replace the diesel filter try purging the fuel system and took it for a drive It for about 3km it was ok then with foot flat no reves and the jeep is creeping.

 

I assume this is an international Ranger, since no North American Rangers had a diesel engine option after 1985.

We don't have info on those engines here.

But if diesel fuel in the tank has been sitting for awhile it can get organic growth that will clog up the fuel system and water will settle at the bottom of the fuel tank.
Generally it is better to drain all the fuel out of the tank then filter it while putting it back into tank.

Simply adding more "clean" fuel won't prevent existing contaminants from clogging things up.