There are actually several reputable transmissions available for our trucks. Hopefully this thread won't turn into a huge mess like questions like this does at other sites.
A few that come to mind are:
Brians Truck Shop
Monster Box Tranny
*note here...there was some talk that production on these has shut down briefly to upgrade manufacturing facilities
ATS Transmission
Suncoast Trannsmission
John Wood Transmissions
There are probably more out there...but most everybody will agree that these are the big dogs for our trannies.
I have heard good and bad about all of them...and they all have differing levels of performance and cost. And they all have very loyal followings.
Research them all...ask good questions and pay carefull attention to the cheerleaders. Many guys will cheer up the tranny they have and poo-poo all others.
Myself...I have the MonsterBox tranny. It has served me well and I tow a very heavy 5'er. I don't put out the HP or TQ that alot of guys do and don't aspire to that level. For me and my uses...it was a perfect fit.
Others hopefully will be along to give advice on what they have.
Oh...and welcome to the site.
red