Staffy

I know this hasn't got much to do with support beams and stuff but its generally about gearbox removal and i'd appreciatge some help. Yesterday i was driving my mk1 1.9 td galaxy when all of a suden the gear lever felt very stiff. I had to stop at a junction and then i couldn't select first or second gear so i pulled away in 3rd. At a convenient place i pulled over and found that 3rd and 4th gear could be selected but 1st and 2nd, 5th and reverse could not be selected, the gear stick was very stiff and i was unable to move it from side to side. I managed to get home and i've had a look at the selector cables and they are fine ( i disconnected them both from the selector assembly and the cables work ok). What might be causing the gear stick to be so stiff and prevent me from selecting gears? Although i can get 3rd and 4th the stick is very stiff and i can hear a grinding sound coming from the selector mechanism on top of the gearbox. If the gearbox needs replacing is this a big job and would the engine need to come out? Although i've never removed a gearbox i'm more than willing to have a go because the car is undriveable and i've got other transport i can use. Any help please....