Transmission issues

[Parmy]

Hi guys, I'm new to the forum and should have come on here years ago. Here is my story, hopefully someone can help with my issue.

I have a 2007 GMC MAX, 1500 4x4 with a 6l and 4:10 gears.

When I bought the truck 4 years ago I was getting about 600-640kms to a tank of gas...winter came and mileage dropped to about 530kms....the service guy said this was normal. I also complained the when I pulled up to a stop, I would feel a clunk...almost like a delayed downshift into first. Summer came and mileage went back to the 600 range. The next winter came and mile ages went to 400-450 km range....I'm now at the 540 km range to a tank. I took it to a tranny shop and the mechanic seemed confused. He said it was shifting fine however he said it wasn't going into lockup...which explained my higher than normal cruise rpm...90km/h is about 1800 rpm....used to be 1600 ish. He watched the rims jump up and down as we drove.....said it was going in and out of lockup. He was confused as to why it was not throwing a code for the converter not locking up. Any thoughts as to why it's not locking up.......the lack of gas mileage is killing me.