I rebuild gearboxes, diffs etc for a living and I trained with Land Rover as an engineer.
I personally went through 7 late 2a gearboxes in 3 years, no i don't drive like a idiot but occasionally use a trailer and load my 88 up to the brim with logs or Scouts .
The series 2a box in real terms has to many places for faults to start. Layshaft, loose fit on input gear into transfer box, rear layshaft bearing is awful ! far to small. A bloke i worked with has worked on Land Rovers since the 60s and he was rebuilding gearboxes that were 2-4 years old all the time. I love the series gearbox its period and great, but the 2a isn't smooth and this causes internal parts to smack against each other like the gears on the layshaft splines, you may not feel it happen but this is my view and i feel its the cause of the issues. A series 3 box suffix D is far smoother and allows gears to mesh faster and more gently. Along with the layshaft being one peice. My best mate has a TDI behind his and its been faultless. but make sure you run 80w/90 not EP90 synthetic gear oil is far better.
Thing is a series gearbox will generally always work unless a shaft breaks or teeth shear off but i bet 75% of people here actually have gearbox faults and not know, but because it was built in the "good old days" it continues to soldier on. The series box is a design from the 30s and originally was designed for 30-50 bhp max. Land Rover made it slightly better but not a lot.
Unlike a series transfer box which is good for 500nm of torque. only rebuilt a few series transferboxes but hundreds of lt230s
V8's are kind on gearboxes as their torque band is even and a TDI's isn't. Hency why the last many years.
Sure people will disagree but i've rebuilt in excess of 200 series gearboxes and 300+ LT77/R380s
And a Suffix F-G-H LT77 is no weaker than an R380. Early lt77s were poor but still better than a series box ,hence why morgan and car manufactures didn't start using them still suffix C when they got bigger gears and bigger bearings. thing is my 200tdi disco did 225k on its original LT77 is that really that bad on miles and the sychro on 2nd went mainshaft was like new. lets see how many modern cars are even on the road at 225k
Needless to say I have a suffix H lt77 defender box with series transfer box in my 88" with a 200tdi VNT and massive intercooler,3.54 diffs, uprated half shafts and 1 ton brakes on the front with a heystee bigger servo. No more worries, tow big trailers and you can quickly pull out of junctions and not have to worry about breaking stuff. I use mine for work and i do a lot of miles a year so my situation is different to most but thats my opinion