Dropped out of gear

Hey all, I have a new to me 2007 GMC Yukon XL 1500 5.3l 4wd. 245000+ miles. I knew I'd be running into issues even when I bought it but the vehicle is so clean for the year and miles. I'm preparing to eventually overhaul and remove the VFM on the 5.3 and overhaul the 4l60. Yesterday on the way home from work, about 35mph up a steep hill, under high engine load the transmission seamed to drop out of gear and was basically in neutral. I had to stop and shift into roll into a parking lot behind me. Then I put in park checked for codes, nothing, then drove home the rest of the way with no issues. A couple weeks ago in my drive way I shifted to drive to leave the drive way but nothing was there , basically in neutral. Shifted to park, cycled ignition, drive came back. I have ideas what it might be but I'm just consulting people with my knowledge than me first. Thankyou

Assuming you checked your transmission fluid level? And how does the fluid look? How long has it been in there?

.... Do keep in mind that at that mileage, if that's the original transmission, it's lived a long and good life, and it's days are undoubtedly short....

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please. If we can't see it, it does not exist.

You are already receiving sage advice from the knowledgeable folks on this Forum.

People who know more about the 4L60E than I do might say something about the pump failing to build enough pressure,
or the valvebody not holding pressure, or somesuch ... at over 245,000 miles on an '07 XL (regardless of axle gears),
whether that 4L60E is on its OE build or its 1st or 2nd rebuild, it's very likely time for another rebuild,
preferably to '4L65' or '4L70' or '4L75' specs as your budget allows.