It very well could be the AFM, sorry, don't know how to test it, but my bud had a problem with his 3.0, replaced the afm and it went away.

Some guy with a 3.4 got pinging when he had alot of carbon buildup in the pistons/head.

Have you tried runnig 91, reseting the ECU and seeing if it still pings?
Maybe your ignition timing is advancing too far, causing the ping?

Good luck...

89 4Runner
3" BL, M/T locks, 33"bfg mt, bilstein, Kayline, tubebumper, toyotafiberglass panels
TBI: Elocker,3.4 w/ORS,b+b,S2Sstg2cams,arias pistons,P+P intake,TRDs/c,URDpullies+7th,downey headers,MAPECU2,WEGO WB, SupraMAF,walboro255,stg4clutch, EPaOo2 sim