That's kinda high for shifting. i rarely get over 3000rpm on mine, and that's going up the huge *** hill I drive every day. If the governor solenoid is an issue, has it thrown an engine code? Mine did something stupid out of the blue and threw codes. Had them cleared, and touch wood, so far it's been fine. If it is that, then from the posts I read, it goes into a limp mode, like you're in 3rd gear from a stop. Not too hard to change or expensive so maybe worth trying.
Since you were in mud, I'd open up the front diff, inspect that everything is ok, clean it out, and put in fresh oil. If you have mud or water in it, can do bad things. Better to do and have piece of mind for the time and cost it takes. Could also be with the mud, it's caused an issue with one of the front axle shafts. Is it specific to the left or right side?
I'm not sure exactly how much play is 'normal' for the front drive shaft, as I've had issues with mine. There's the rotational play with it on and in 2WD, and when it's in 4WD. I don't know what it should be at. The shaft itself, I've had issues with the rear rezeppa joint and the boot being ripped. Also there is what I was thinking too much play in the joint, although it wasn't that much. Now with the donor I put on to keep it working since I don't have the $ for a new shaft right now has been working great. Boot was in great shape and has held up so far without problems. (over a month now) The one thing I did notice though, is that the joint was very tight, as in it performed as it should, but didn't find any play in it at all, sideways, or in and out, if you pulled on it. Hoping it'll hold up for a while.
The beatings will continue until attitudes improve!
HID Quad bi-xenon + foglight retrofit