whenever you have the problem of not being able to go into reverse, its 90% the transmission, and i further believe that because you said at 50mph you were going 4k rpms..... thats another 90% transmission problem, or tranny related part. (remember, im talking fords here) but of course i havent driven it and havent experienced the problems on my own so i cant say for sure. as for your questions, you're delving into stuff i just usually take it to the dealer for, so i can be of no more help.... but good luck tho.