Anybody here have any recommendations for gear/transfer box repairs in North Hants (Aldershot)?

Had to use "old faithful" (LWB 7 seater Isuzu Trooper 3.1D) to go to Leics on Saturday to see my daughter & the grandkids. Doing approx 65 - 70 and at about J17 on the M1 the car appeared to jerk slightly as if it had been grabbed from the rear and "snatched" backwards.
This was only a momentary blip and I carried on the journey as everything appeared normal.

A minute or so later I have total power loss as if the gearstick had been knocked in to neutral.
Pulled over to the shoulder and after a bit of messing about found I had drive in 4th gear, but only when 4WD (High) was engaged.
I managed to get the car rolling again and complete the journey (at the expense of the clutch judging by the smell of it afterwards).

Now for the last 3 months or so this car has had a very slight whine coming from the box when it is in neutral, but this would stop when the clutch was depressed so I put it down to the mainshaft bearing and didn't really worry too much about it.

The current symptoms are thus:

2WD selected - no drive in any gear, but no "mainshaft bearing" noise either when in neutral
4WD (High) selected - drive available in 4th gear only, again no noises in neutral.
4WD (Low) selected - no drive in any gear, and gearbox sounds like it's smashing itself to bits internally when in neutral (depressing the clutch stops the noise).

Now while the car itself is quite old (1995), it has only covered 115,000 miles from new and has always been serviced on time.
I don't particularly want to get rid of it as it has been incredibly reliable to date, can legally tow an Omega on a trailer, and it has the required amount of seats to get get my current family in as well as my daughter & her kids at the same time.
I have looked on ebay, but there is nothing available spares wise that matches (or even one being sold as a working car that matches the current spec) so my only option is a gearbox rebuild.

Any and all advice would be most welcome.

Sassanach has a GM gearbox fettish (he's in trowbridge). Not sure if he could offer advice?

Though I guess with your treatment, heavy lifting is quite tiring!

I'm sure there used to be a transmission place in North Camp. No idea if they're any good or even still there, mind.

Went to a place in Fareham once where a member got her auto box refurbed. Mostly auto boxes but he might do a manual, I suppose. He did the job on that occasion for reasonable money, and had about 3 auto boxes completely stripped on his bench and didn't appear to be fretting about getting them back together again. I would have been. 

Given that something fairly fundamental has clearly given up it begs the question of whether a 2nd hand box would be wiser. Then again, it could be something trivial, and a 2nd hand box would be a complete unknown.

Hmm, almost the same? had a mitsubishi shogon in which would only drive in  high range 4wd anything else it just locked up, after checking everything i could was just about to pull transfer gearbox out, when i noticed the rear prop was turning freely in spite of it being in gear? turns out the rear diff had broken! when stripped the main bearingcap was shattered and the pinion was abe to miss the crown wheel, so no drive t the rear, but it would drive in 4wd ,
think in your position i would jack it up and check, diffs gearbox and transfer box just to see whats playing up? then make a dicesion