Can you supply a serial # for that TW,

As I recall it had a Barnes pump installed.

No adjustable pressure reliefe installed in that Vickers valve.

Will the unit tilt a full load?

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Doc

I've got one of those "spring chickens". Does it lug down the pump motor when you strain it? If not the pump may be worn enough so it can't make 2000 psi like it's supposed to. There doesn't seem to be a external pressure adjustment on the valve or pump. One of the two plugs under where the inlet hose goes into the hydraulic valve is the pressure relief. You're going to have to take it out and try to shim the spring some.

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)..

its rated at 2500, it should lift it. Its a keg of roundup that I need it to lift.

Check your capacity plate it should tell you the lifting capacity. Also remember that it is rated at 24 load centers. If the unit you are lifting might be to long. The rated capacity drops dramatically