I have a problem with my transit mixer if not loaded the drum rotates normal but when it loaded the drum wont rotate and the hydraylic hose from hydraulic pump to transmission pump vibrates....
You need to do some testing with hydraulic pressure gauges. Either the hydraulic motor and/or reduction is shot, or the pump and/or relief valve is not producing enough pressure. It wouldn't hurt to check the bearings that support the drum, and drain the lube from the gear reduction to see if there's any obvious problems there.