No lock up

problem: towing a small utility trailer(approx 1500# gross), going uphill transmission down shifts to 3rd. when at top trans shifts back to 4th and lock up. After about the 4th or 5th time the transmission stayed in 3rd. Never went back to 4th and lock up. Continued my trip in 3rd gear. Next day trans shifting ok. Anyone know what happened?

If it misbehaved as you described it likely set a code in the computer.

Get the codes scanned and look for things related to the transmission. It could be a sticking shift solenoid.

Start with the simple stuff too, make sure you're topped off with fluid first.

J