Allison trans question

I have a 2007 Classic CCLB SRW LBZ 3500 with a bit over 340k on the stock trans. I have been noticing a shudder/shaking and what seems to be very minor slipping when I pull off from a stop even when empty. This shaking stops when up to around 20 mph but it happens every time I accelerate from a stop. I checked the fluid and it is bright red, at the proper fill mark and doesn't smelled burnt. I am wondering if I should start considering a rebuild or replacement transmission. My truck doesn't have any mods other than a CAI and aftermarket wheels - never been tuned. It doesn't see any heavy towing, but I do pull a 7x16 enclosed toy hauler with my Harley or zero turn mower in it on short trips.

Any suggestions from the more experienced Duramax / Allison folks on easy fixes or should I start shopping for a trans?

I would like to be proactive on a fix and not have any unexpected down time / get stranded with the truck.