Just to summarize the conversation in the chatbox for anyone that runs into this later.
The RPM should drop, but not all the way to idle. You can get the effect you're talking about if the splines on the 4th clutch hub are stripped off. You also said you're at 3000 RPM at 70 MPH, which is far too high, and supports the theory that the clutch hub is toast.
While cruising on the highway, pull the shifter back into 3rd and see if it downshifts. If not, we've found the problem.
Thanks to Reptile for helping in this diagnosis.
