Hi there I recently purchased a 93 Chevy c3500 1ton dually with a 350. Truck runs great and has had a transmission rebuild roughly 30,000 miles ago. 1st, 2nd, and 3rd work as they should, but 4th will not engage. When you are in 3rd and let off the gas it idles low as it should as well. Any thoughts would be great! Thanks!