98 dodge drives me nuts

i drive a 98 dodge 3/4 ton 4x4 and when it is near or below freezing temp outside, my truck will not shift into high gear although it keeps trying if i push it a bit. Funny thing is that in the morning it does this but then after work in the evening on the way home usually is fine. I had it in the shop and they put a new tranny in it that seemed to fix the problem for about 3 months and then came right back. it drives me crazy going 70 or 80 km/hr on the highway. also, i dunno if it is a related problem but it also stalls out if i put it in reverse with my foot on the brake sometimes. anyone with any ideas would be greatly appreciated. thanks