Possible F350 Transmission Trouble; Fixes

We decided to take our truck on our road trip and
stopped off in South Dakota to see Mt. Rushmore.
The steep hills leading to the monument brought to
our attention that the truck was having trouble
keeping power. Once we got off the mountain and
we're back on the road it was worse. We managed
to pull off and stopped the truck but it wouldn't
restart. The mechanic we had it towed to said our
started was completely gone and that we had 4
fuel injectors either weak or gone. So we fixed all
of that and got back on the highway for home. All
was good until we got above 70mph for the first
time. The truck lost a some (not as bad as before)
power and had some high vibrations. So we aren't
taking the truck above 2000rpms/70mph. Is this a
transmission problem? Would a flush and change
of fluid help?