Sounds like clutch packs it its slipping that bad.
Before whacking new packs in, some recommend checking the fluid level, and replacing the filter/fluid first. Also, the TPS apparently has a bit to do with the gear selection/slipping. You also have the TV (throttle valve cable) to adjust.
I would do the simple options first..
Luckily you live in a country with lower labour costs.
I guess if the tranny doesnt come good with the smaller suggestions, then a box from a lower k's wrecked Jeep will be on order.. or a rebuild.
As you know, in Oz, i don't think you would get much change from 2K. Hopefully in Thailand it would be much, much cheaper..
http://www.fourwheeler.com/how-to/tr...ssion-rebuild/
Cheers,