There are pressure relief valves in/on the control valve, but the fact it can tilt the load (can it lift the steer wheels off the ground if you try and lift more than 2.500LBs, by tilting?) indicates to me, that at least on this truck, the control valve pressures are pretty close to what they should be, (but the right way to know is pressure readings with a good gage).
You most likely have a set of worn seals in the lift cylinder, or possibly even the cylinder itself is worn out, as this unit is no "spring chicken", another likelihood is that the hydraulic pump is getting worn out and not able to produce as much pressure as it had in the past, (and/or all the above)..