To measure what the rpm difference will be w/30's and 4.88's go hereDrivetrain Calculators, and plug in the numbers you have now. You can get your transmission ratios here W-Series. If you have G58 instead of the The W56 transmission, I believe the gear ratios are the same. Marlin used to have the info on his site, it might be there in the tech section.

Most of the long time Toyota wheelers would rather pay for original stuff from Marlin than the cheaper stuff from Trail Gear.

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