My 90 GS Custom recently stopped moving.

While in Park I can floor the gas and get plenty of response, but once in drive absolutely no acceleration and RPM's will not go past 2100

. My mechanic linked it to a fuel injector problem which I have had before with this car.
This weekend I am going all out and replacing all the injectors plugs and wires

. Just wondering if anyone has ever had this happen or hasdone this and knows any easy way to get to the three plugs on the backside of the engine. Any tips on the fuel injectors would also help