Expand the cylinder so the shaft is fully extended, using a hydraulic pump.

Weld a 3.5" piece of 1.5" DOM roundstock inside of the tubing at the end of the shaft

Take care that the top of the shaft doesn't get hot; there is a rubber seal between that and the cylinder that will melt and cause leaking if you're not careful. If it starts getting warm, dunk the shaft in water to cool it down.