1999 Chevy Truck Transmission slipping

I haven't had any noticeable issues with the transmission until this point. I drove it to work one morning, then on my trip home when I started it up, it hesitated to pull off in reverse, then even worse going forward. It drives extremely sluggish. The engine runs smooth, it tachs up but the truck will hardly go. It's this way in forward and reverse. It slips in and out of every gear and at one point the engine tached up at 2300rpm with a speed of about 45 mph driving down the highway. I finally parked it and had it towed home. The fluid appeared clean, no smell and was full. This appeared to have come on all of a sudden. Would a fluid and filter change help? I bought the truck used with a 167,000 miles on it four years ago. The original owner has had the engine replaced and the tranny rebuilt once before but I don't know when. I've never changed the transmission fluid or filter in 55K miles. Thanks for any insight.