4L80E problem, can't find answer

[CR506]

I've got a 98 K3500 that had no reverse. I rebuilt the trans and then when starting accelerating from a stop the truck felt like it was bogging down & had no power. It had the feeling like a parking brake being set, but it's something in the trans. I pulled the trans out and had it rebuilt again by someone else with tons of experience rebuilding transmissions. Put it back in and same problem. The trans has all new internals and electronics. Transgo HDII kit was installed. Still, whenever taking off it's like something is holding it back then once you finally get up to 30mph or so it shifts ok to 3rd & OD. Reverse works fine & feels normal with normal power. Forward you can't even spin the tires in wet grass. Any suggestions? Would a faulty brake switch cause this? Also, a new torque converter was used on each rebuild, so that can be eliminated.