eddiekimberly

1996 k1500. Ran great until this evening. Pulled a trailer home last night, about 20 miles. 10' lawn mower trailer with 25 piece of 2' wide, 12' long pieces of HardiPlank siding. Nothing too heavy. Today, I went into town, then as I was leaving Hardees, I started up a hill and was in 3rd gear, I think. It happened kind of quick so I didn't have a chance to absorb quite all the details. I THINK I was in third. Suddenly, it kicked back to second, then back to third, back and forth 3 or 4 times. Also the speedometer started going up and down considerably, 10-15mph, with little apparent change in vehicle speed. It straightened up after a few seconds and was fine for a few miles then started again, exact same thing. I pulled it out of overdrive into 3rd and it straightened right up. Never had any engine or transmission issues in the 6 months or so that I have had this truck. Anyone have any ideas as to a starting point? Personally, it sounds like a ECM issue out an overdrive solenoid to me, but my experience is with a Dodge Ram. Also did a bit of further research and it sounds a bit like a vehicle speed sensor. Only CEL code I am getting is for an O2 sensor that I haven't quite gotten to yet. Also, may be important to note that as I was parking the trailer in my wet *** yard, I had to use the 4wd, both hi and lo ranges. I have used the 4wd several times on it with no other problems. Ideas? Thanks in advance.