99 Blazer losing RPMs at low speed

99 Blazer 4.3 V6 auto trans - at speeds of 10-25 mph my rpms are dropping. Once I reach 25 mph it runs great. I've had to 2 mechanics check it out. First one replaced the fuel filter, which didn't fix the problem. So I took it to a Chevy dealer. SES light was on and they diagnosed that it needed a new transfer case control motor. But I wasn't about to pay them what they wanted to replace it. I purchased the TCCM and had a friend replace it. Then I took it to AutoZone and they checked the codes. Still said TCCM. The guy at AutoZone reset the computer. SES light went out for about 25 miles. Now it's back on and truck is having same problem. He suggested I might need new plugs and wires. So this is next on the list. Just wondering if anyone else has had same problem. Other possiblities?

Well I've taken her in to her third and hopefully final mechanic. Since she has 74,000 miles on her, I figure she's due to have new plugs, wires and an O2 sensor anyway. Hopefully this will fix the problem.

Losing my patience and noticing a significant drop in my bank balance. New plugs, wires, O2 sensor and overall tuneup....$440. SES light is off and no trouble codes on the computer. Any ideas?