Don't take your truck to the dealership to have the transmission looked at unless you truck is under warranty. Their answer will always end with you spending a lot of money.

Find a good independent transmission shop. If you don't know of one, ask your nearest 4 wheel drive parts store for a recommendation.

In 2002 my old 1995 F-150 4x4 started leaking transmission fluid, a lot of fluid. The local dealer recommended a rebuilt 4R70W and highly recommended checking the transfer case. Total bill would have been over $2000 if I had let them do what they wanted. A local shop checked it out, found a gasket that had failed, fixed the problem, flushed the transmission (and converter) and replaced the filter for under $200. That transmission was still in the truck and working perfectly 150,000 miles later, when I traded the truck in.