1984 Cadillac Deville Transmission not going to lock-up

My 84 cadi deville has been running great. The other day while traveling on the t-way I decided to try the cruise control for the first time. I just bought the car two months ago. The light came on and I pushed in the button for the cruise and nothing happened. I tried it again and still nothing, no big deal. When I came up to my first red light after this the car went thru the shifting 1st 2nd 3rd drive then lock-up as soon as it went into lock up the rpms rose several hundred and a rumbling noise started to shake the whole car.I dropped trans into 3rd and noise went away, if I put it back into "d" instantly to rumbling and rpms went up. I refuse to believe its the trans, Could the cruise have done it or a vacumn line or pinched cable? Thanks for your help. Derk.