97 Dakota 3.9L transmission slipping

[fastlife1]

I have a 1997 Dodge Dakota extended cab with 36K miles. I recently had my transmission serviced by the dealer with fluid and filter change.
I went out this morning pulling my boat to the lake with no problems. When I went to pull the boat out of the lake at the ramp, I would pull up about 5 feet and then the truck would just slip out of gear. It did this 6 or 7 times before I could get it off of the ramp.
It was very cold this morning, but don't know that that would have anything to do with it.
I let the truck warm up a little and and then proceeded to head home, about 30 miles and didn't have any problems with the transmission slipping.
Any ideas on what the problem might be?

Thx, fastlife