1991 2500 5.7 problems

lately I've had a problem with my truck trying to hold a steady RPM. After I drive a while and I'm on the highway (usually) the RPMs drop off and the truck wants to die. If I stop while it's acting up, the RPMs will drop off and jump back up. Also there is a rubbing/grinding sound comeing from under the truck..I can't pinpoint it, but I have a good feeling it's something to do with the driveshaft somewhere...any ideas would really help.