P0871 is a pressure code for the valve body. This is most commonly seen in trucks that have been tuned with the line pressure turned up (usually 250psi or higher), but it has happened on rare occasion on stock trucks that have been worked pretty hard and used to tow at the limits of the truck. If the trans has been running hotter than normal, it could in turn damage the valve body.
P0735 is Gear 5 incorrect ratio code. Again, typically seen on tuned/deleted trucks that have been turned up. If not, the other options are a bad shift solenoid or a faulty harness or connector.
P0729 is Gear 6 incorrect ratio code. See above.
OP - questions for you:
Is (or has it been) this truck tuned/deleted? Even if you add a 75 hp tune, that is roughly 150 lb/ft torque increase. A 68RFE won't like it.
Are you running much larger than stock size tires? Maybe 35"s or 37" tires?
What is the transmission operating temp? You should rarely ever see it crest 190 degrees even if it's scorching hot outside and towing a heavy load. Usually you'll see somewhere in the 165-175 degree temp range.
Is the fluid at the proper level, and what color is it? When was it changed last?
A bad valve body "could" cause these other codes if it has not been tuned, so it doesn't necessarily means the trans is toast. But you definitely need to get it corrected time now.