Transmission Or Spark Plugs

I have a 1999 Trooper and changed the Spark plugs out last week to the Bosch Platinum 4+. Seemed to be fine and today the transmission starts messing up. It is real jerky and at one point the Gear display light switched from Drive to Neutral while I was driving it. After that I stopped and turned it off and restarted it. When I shifted into gear the Gear light jumped all over the place back and forth. When started to drive again it shifted really hard and when slowing it downshifted really hard and jerky. Is this problem related to the spark plugs at all or is it just a coincidence? Also could the tranny problem be the transmission range switch or some other computer problem? Thanks for any Help.

I wouldnt run bosch plugs in the 3.5L, but you might be ok with them in your 1999. They definately would cause issues if you had a 2000-2002. Next time use denso or NGK plugs. Isuzu's like these best. Your trans problem is a different issue. Funky "PRND321" dash lights, accompanied by harsh shifts & sometimes a "check transmission" light is a bad transmission range/mode sensor. It's a little black box attached to the shift arm sticking out the left side of the tranny (drivers side). It tells the PCM/TCM which gear you have selected. You could unplug it, remove, disassemble and try cleaning it. They either get too hot (being near the exhaust) which melts the dielectric grease in it, causing problems, -or- they get moisture in them causing problems.
G/luck
Joel

Do you think I should change the plugs back before I might get a problem or would I have already seen a problem in the first week if it was going to happen? They seemed to be doing good and I got 16.5 MPG on the first tank of gas ( which is pretty good for a trooper in the city).

Nah.. leave'em in unless you get random misfires or related CEL's.
G/luck
Joel

sound like you have a problem with the trans range switch it will set a code but will not cause the engine light or tranny light to come on