2001 dodge 3/4 ton van trans. feels like somthing is a wry.?

recently after a 13 day period of hard freezes here ne fl. i got in my dodge van,and noticed slow to take off.has an overdrive setting on in shifter knob.drove for the day to see what would happen,engaged in overdrive,over drive off noticed similar movement in both settings hesitation and slight throw in gear as reached shift speed,then manually put in 3rd(at tran)then into drive and had good torque,but can not do this for long.problem i do not,should service,flush,replace,or rebuild van has 87000 bought used 2 yrs.ago no trans service as of yet,the dealer i purchased from repair a seepage around pan with sealant i ume filter replaced at that time,but cant say for sure .did put a synthetic trans.fluid back in reccomended for chrysler s/dodge,any comments ,thanks tony