I wouldnt say its actually slipping, just the ineficiency of a stock convertor, they have way more slip than a suncoast. and stop and go and under 35 keeps it from locking. me and russ both battle that with our autos and 3.55 gears.
even when I had my 00 I plain out hated the TC and trans, even had Bill try writing different lock up and shift stratagies. never did get a good all around one.
then I put the Suncoast in and the transgo kit and that made a world of difference, pu 50k more hard miles on the truck with no trans problems.other than an o ring going bad in the Tc.
But anyhow to answer the question, yes ya can put a lockup switch in, or even get an autolockup from a few companys.
the downside to locking at low mph is it will shift hard if your on it, and if ya allow the engine to lug it will stall, or use more fuel, and ya must remeber to shut off when ya stop. also if ya have the Tc locked and its pulling to hard ya can actually slip the tc when its locked and that is bad.
30k its due for a fluid change also, try merc V and that may also help,
other possibility is the trans is on its way, 150k on an original.
Stick a fork in her, she's done.
