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Mt son has a 1997 ford F150.  The transmission only works when it is cold or at low speeds like 5 MPH.  Once you get out on the road it will quit completely and will only start working again after it sits for awhile.  Here is my problem.  He is in the service and most likely headed to Iraq in a month. He has the skills and tools to trade out the transmission for another one but I would need to find one while he is gone.  My other choice would be to take it to a repair shop and have them complete an evaluation on what is wrong to see if the entire transmission is bad.  There is no way he could afford to have them change out the transmission. Bottom line, is the evaluation worth the cost or should we just bit the bullet and find a used or rebuilt transmission and change it out  Thank you for you help.  You guys are great. (PS) Should we also change out the transfer case.  

It sounds like your filter is clogged.this usually happens because something is coming apart and putting debris into the system which finds its way to the filter and blocks fluid flow.Drop the pan and change the filter.Maybe you will get lucky and it will run good for a while.