UK Car Repair Questions? Ask a Mechanic for Answers ASAP
It sounds like the system has dropped into fail safe which results in a large drop in power and loss of low gear/torque (the difficulty going uphill).
As long as you are not getting any of the following..... flashing check engine lamp, heavy/billowing smoke, stalling, very increased engine temperatures.... you should be OK to drive it. If there is any change in those conditions however I would strongly urge you to have it towed.
The engine temperature is likely to increase a small amount, that is OK... as long as it does not get very close to the hot end of the gauge you are alright, a 10-20% increase is expectable though. If you can try to minimize your lugging on hills etc.... the longer you can keep it at speed/cruising the better. If you have a hill approaching try to build speed on the approach when power is not as demanding then use the increased cruising speed to help up the hill. If you have to lug the truck to get it up the hill this increases the stress on the transmission and transmission fluid.