1998 Chevrolet Blazer engine misfire

I have a 1998 Chevy Blazer with the 4.3 Vortec engine. I have had it about a year and it is in very good condition and has been taken care of very well. About 2 weeks ago I started having transmission issues. On the interstate if I started going up hill if the transmission down shifted it did not want to shift back into overdrive. It would seem to be stuck in the lower gear. I do not have this issue except on the interstate at around 60 mph or faster. If I am on the normal roads the Blazer runs excellent. I then realized that just before the transmission would feel stuck the engine was running rough and my service engine light would come on. I had the codes read at a local auto parts store and I am having cylinder misfires. I have since then replaced the plugs, plug wires, distributor cap and rotor. This did not fix my problem. I have not found any vacuum leaks or anything else visibly wrong. I have been working on cars most of my life. Everything from oil changes to rebuilding engines but this has got me stumped. So any help or advice would be greatly appreciated. Thanks.