transmission 95 yukon

My 95 yukon transmission started kickingout of higher gears clear down to 1st or 2nd gear after about 3 miles down the road.It is an automatic,and works great for about 3 miles.This has high miles on it.Could it be the computer causing this,or is there a sensor in the tranny that would do this?

You need to get it on a eng/trans capable scanner and check it out.
If any codes post back codes.
MT