lift and tires

[1990pu]

relatively new to the scene here. younger in life and trying to learn new tricks of the trade. i've got a 1990 pick up, 5spd 4wd and i'm looking to put new tires and rims on it. now, i'm not ready to do a suspension lift and i was wondering what the best combo would be for mudding/offroad would be with the stock configuration. looking to use 15in 6 lug rock crawlers. what would be my best tire with 3.75-4 in of back space? any input on possible combinations for rims/tires. thanks for your input, anything and everything to help a novice out would be much appreciated...

thanks

[Tercel GTS]

Boss had no lift on his 92 with 31x9.50. Seemed to work pretty good with no scrubbing. We don't have rocks here, but plenty of mud in the swamps.

[1990pu]

were these on stock rims? or perhaps if they weren't do you know what size they were and what the back spacing was? i've got michelin lt235/75r15's. which are roughly 29 inches. any ideas on what size bfgoodrich tires of this model would be best? since i really don't know much about it...here's the link:
http://www.bfgoodrichtires.com/bfgap...erraintakm.jsp
that is what i'm looking to put on it, but would any of those sizes fit the stock rims or possibly rock crawler 15x8's? and what's my maximum width i could put on either of these rims?. any input and/or mocking of me for being undereducated is welcome...

thanks again for your help.