need some help with my 4R100. my late 99 f250. Torque converter i think is about to take a ****. The OD light isnt flashing yet, trans isnt slipping, it has only 168k on the truck, trans is unknown. I was experiencing the marbles in a tin can sound since ive had the truck (annoying but i delt with it) but the other night it wouldnt shift out of 2 into 3rd it was at 35 mph at 2500 rpm. then it decided to shift. it hasnt missed a lick since but today when i drove to work it started to make a choo choo choo (Choppy) sound when in gear and it would get a little louder when you would back up or move forward, worse in reverse. I ended up having to leave our shop to a job site and when i got to the job i put my truck in reverse to back up and it didnt want to go into reverse until i got the rpms up higher and then it would engage reverse. if i let off it would kick out of reverse. i noticed the other night backing up that it was slow to engage reverse. anyway back to today. when i pulled up to the job before back it up into a spot, i noticed the marbles in a tin can sound started to go away as i was pulling in, and thats when i tried to back up and it wouldnt engage reverse very well. then when i went to leave, for ****s and giggles i put it in reverse and it engaged put it in drive and it engaged. drove like normal truck was smooth, no rattle. as i got to the interstate leaving a stop light it shifted a little harder then normal into 3rd, and lock up was just a little more delayed then normal. after my 40 minute interstate drive, stopped at a stop light, everything normal and no rattle. takes off fine shifts fine almost all the way home with a slight harder shift in 3rd, to 4th. got home put it in reverse, engaged right into reverse. would it be safe to say its the TC or something else. maybe the valve body, or plugged filter? i know it has a pan drain plug, so i kind of wonder if the fluid was changed but who ever did it didnt drop the pan and change the filter. i have checked recently and fluid is full and a nice clean light red color. any suggestions would be helpful. the truck is bone stock besides a 6637 air filter.