2000 Isuzu TF brakes one rear wheel locks up and front are poor to being dangerous

Right here goes i purchased this early TF just before xmas, its a 3.1 TD now a few days following the purchase (ebay) i noticed the nearside rear wheel locks up when first used normaly only the once but it does it evry time. The front are very poor and seem dangerous to me but it is my first Isuzu. i had them looked at by a mechanic he took the wheels off and checked the brakes to ensure both rears were adjuted up correctly but it still does it and is bloody embarassing when your wheel skids in the high street at 20 miles an hour not to say a tad dangerous. someone mentioned it could be the rear load compensator by the mechanic didnt seem to think this was the problem. any help would be great as i feel i cant use it as it is. the bit under the rear bed Compensator is £500 why is it everything on an Isuzu is £500 got a leaky seal on the rear axel which costs £4.80 but cant be put on as the bearing needs to be pulled off, the isuzu garage said they never come off ANd the halfshaft is you have guesed it £500 plus another £500 for the other bits required and the labour. Any comments anyone or do you know a man that can.
LOL