95 Burb transmission noises and shifting issues

I have a 95 Suburban 1500K 4x4 with the 5.7 L engine. In the past year there is a lot of transmisssion whine and chatter while shifting --seems to take excessively long to shift into 4th gear between 40-55 mph and shifting up and down at those speeds excessively Dealer has looked at it 2 times and says is fine. This weekend took out trailer and problem occurring more frequently. Power seems OK. Any thoughts would be appreciated.