1987 pick-up grinding noise when driving

My 1987 toyota pick-up has 275 000kms, 5 speed manual transmission, 4 cylinder 22R carb motor, and for the past couple of months everytime i accelerate i get a grinding noise. Its at its worse when i accelerate and when i back up. When downshifting it also makes the same noise. The problem has been getting worse (louder), but its never constent. I've tried driving differently (Harder, smoother, no downshifting, etc). Even when driving along, with no speed change, the noise comes and goes. it sounds like the driveshaft may have something rubbing against it, but i cant see anything. Could it be tranny related??

Thanks