Yes the fenders will fit, but they wont look right. The 4x4 fenders are about 2" wider than the 4x4's. If you have to buy the 91 outright and it is cheap enough to do so, you could try them and see what it looks like. If you don't care for the look rebuild the engine and sell the 2x4.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed