OT but need help -1998 Tranny Question/Problem

[Old Chevy Lover]

I posted in 88/98 forum, but know there aren't as many people as there are here. My 98 Ext cab 1500 350 totally stock 60,000 miles... drove it for a little over an hour straight today, not pulling a trailer, not hauling anything, in OD, and I noticed the 1-2 shift getting VERY stiff. The 2-3 shift was firm, and it quit going into OD. By the time I got home, it felt like I had a full race shift kit.. Obviously it was heating up, but does anyone know what would cause it??? After sitting and cooling off for awhile, it was fine. Didn't drive it long enough to heat up again. Was normal driving conditions, some highway, some country road driving. I do not have an extra tranny cooler on it, but it still shouldn't do this under normal conditions.. HELP. I would like to figure it out before something big happens and it costs me a whole tranny!