tranny issues

l have an 89 sierra 4x4 with a 4.3 engine, l only use it in to plow snow lately if its cold it wont move until it idles for 10 mins. at least and then it wont get reverse for another 5 mins. it hard to plow snow without reverse, it works fine in the summer, yes the tranny fluid level is okay . any help is good

You are due for an overhaul, internal seals are bad.