It does sound or me as if one of the clutch packs has lost the linings off the clutch plates and has locked the clutch pack up it is a fault I've come across a few times on the older Hyster transmissions (but usually caused by the wrong oil being used) and will probably need a transmission strip down to rectify. A good few years since I've had this problem but the last one I did was from a stone yard in Keighley with exactly the same problem and the problem I've described. Not a massively expensive job parts wise but labour intensive unfortunately, the engine will need to be pulled and the transmission front plates removed to get at the packs