2008 CHEVROLET SILVERADO K1500 Ext Cab 5.3 W 4X4 – 134K Miles

Something strange happened last week.

While parked in a campground with a slight incline, front of the vehicle facing the incline, I started the truck and let it run for a good 7-10 minutes to warm up, placed the shifter in R then N then D, waited for what I thought was long enough for it to engage in D (from N) and then released the foot off the Brake; all normal activities up to this point. However, when I released the foot of the Brake, vehicle drifted back a couple of feet then .... BANG (like I just backed into something or someone hit me from behind), then a clunking sound as the vehicle engaged in D. I decided to continue to drive forward, slow as we were exiting the Campground anyways and on our way to one last hike on this trip. Vehicle drove fine and all forward gears shifted as usual; I thought I was good. When I arrived to the trail head, I attempted to parallel park but quickly noticed that R gear would not engage. Shifting the gears from back to P to R to D back to R multiple times did not change current state and/or resulted in a favorable outcome. I decided to drive straight home as I was roughly 100 miles away. Driving home, all forward gears performed as expected and vehicle shifted as/when expected. Got home fine,.placed the vehicle in P and turned it off. Decided to turn vehicle back on,..(maybe that would do something) and while shifting from P to R heard some loud clunking (metal on metal sounds) and the vehicle engaged in R. Shifted the gears from back to P to R to D back to R multiple times as well as moved forward and reverse a little and it appears to be back in order; well sort of it. Vehicle is equipped with 4WD,.. I’m able to shift between 2/Auto/4H but I’m not able to engage 4L. Light keeps blinking.

For historical context, vehicle has not been used for towing and not abused in any one particular way like hard driving; it has been primarily a daily driver. Transmission fluid has been changed recently and is full. 1-2 Transmission accumulator and A/B solenoids were replaced as well; about 3K miles or so. At no time did any warning lights came on. Sometimes, shifting from P to D can be a hard shift/engagement, although sometimes not. I have noticed that I can lessen this hard shift by letting the vehicle run for at least 5 minutes before driving off,.. specially if vehicle has been stopped for a little while; not so much of a problem for short time stops.
I will most likely have to take it to Mechanic for an assessment,..but I’m interested to see if anyone has experience anything like this in the past and/or what am I looking at for repairs. I’ve only driven the vehicle around my neighborhood as I’m trying to gauge what my next steps should be and/or what I’m up against. Any thoughts or comments are appreciated.