Failed axle seals in Toyotas of this vintage are not unknown, but I’m not sure that they’re any more common than in any other make or model. And you’re right, replacing the seals in this model requires an awfully big hydraulic press and some pretty specific tolerances to be observed.

But the bigger question is why the seals keep failing and, more specifically, why two seals would fail simultaneously. Often, an axle seal will fail because of wear or play in the wheel bearing it helps protect. But to have two to fail together (one being a new-ish one) suggests a problem with the rear axle’s breather system. This is designed to allow pressure to escape as the axle unit heats up and internal pressures rise. If the breather isn’t allowing this to happen, the pressure can increase to the point where it will cause the seals to fail. So that’s where I’d start looking. A blocked breather fitting or crimped breather tube could easily be the cause.

It’s pretty unlikely that a 1987 Toyota van even has a limp-home mode. These were pretty simple engines and shouldn’t really tax a good mechanic too badly. Provided you have spark, fuel and compression, an old Toyota four-cylinder engine is more or less obliged to run.

If you’re happy that the carburettor and ignition are spot on, the next step is to perform a compression test to see if the engine is internally worn out. This is a possibility and would explain the sluggish performance. You should also make sure things like the air filter and fuel filter are clean and not restricting the flow of fuel.

There’s a very good chance that your problem concerns the cable or linkage that connects the gearstick in the cabin with the actual gearbox. It’s very common for one cable or linkage to operate half the gears, and a second linkage to operate the rest. If this linkage has fallen off or become detached somehow, there’s no longer a physical link between the gearstick and the transmission selectors for those gears. At which point, it’s impossible to select the affected gears.

A quick check under the gearbox will confirm whether this is the case, and fixing it should also be a simple case of reattaching the linkage or possibly replacing a broken cable.