The B series Frontera has a reputation of failing speedo cluster. Its a relativly simple job to fix once you have the dials out and your handy with a soldering iron. Some people report that they can get it working again by switching the sidelights on and off again or just leaving them on. There are also people out there that offer a repair service as well, for not bad money. Crux of the matter is it doesn't affect the driveability of the truck at all. All the 4wd is independant of the speedo, you just won't know how fast your going.
As a point of note the Frontera doesn't have "overdrive" just normal 5 speed box and a reverse. There is a button on the dash that activates the 4wd using a system of vacuum pipes and essentially pneumatic cylinders, controlled by a small ECU built into the dash under the ashtray. No centre diff as well (as found on the Land Rover). Transfer box bolted to the back of the gearbox, that has a prop to the front diff and then your axles to the hubs. The nearside drive shaft is split and when 4wd is engaged an actuator connects the transfer box to the gearbox, and another actuator joins the two halves off the nearside drive shaft together giving you 4wd. Front hubs on the B series are fixed, though people often replace the hubs with manual locking ones allowing for more off road control and better economy as the wheels are effectivly free wheeling now. Advised not to angage 4wd on tarmac or grippy surfaces and only use it when needed, as the 4wd system on this is locked and all four wheels turn at the same speed. That means you can get torsional wind up in the system when in 4wd and going round corners as now the outside wheels cannot turn faster than the inside wheels. Does mean that you don't get loss off drive when you really need 4wd though.
A further point is the Frontera uses the Isuzu Trooper 4wd system. Vauxhall just designed a different body to go on the chassis effectivley. There are differences like prop lengths and locations of ancillaries but a lot of the parts are exactly the same made in the same factory.