Auto Trans question..

About 2 weeks ago my 93 started to shudder a bit under light throttle when in o/d,lockup from about 40-50mph.If I increase throttle but still stay in o/d,lockup it goes away.Over about 55mph I can't make it shudder.I also can't make it shudder in 1,2,or 3rd.Any ideas? Other than this the trans.performs PERFECT!..Any idea if this is the end of this trans.,? or how long it will last? If someone has had this condition and the tranny packed it in soo after..that would be good to know in advance! I drive it pretty easy..but don't want it quitting in the middle of nowhere..in the winter.I can get another good used tranny for about $200..have that one installed for about another $400..or do it myself.Or I can get another trans. and rebuild it myself for another $150 or so..before putting it in.Not sure of the mileage on this one? But it has been well maintained by me for a few years. I did phone a few transmission shops..one seemed VERY HONEST and REAL QUICK to the point.I am in Toronto..Escort haven.MAN!! there are a LOT!! of Escorts on the road here.! This guy said it is the 3-4 solenoid acting up and feeding mixed/wrong signals to the computer.There are no codes yet..but he says when it gets worse..out of range..it will code.He says he sees this all the time with these trans.I dunno..seems to make sense to me..any thoughts? I don't have a transmission manual and don't know how to test it?Other than trying another one.Thanks for any help/info..