1999 Blazer LS - 153k miles.

Here's the issue I'm trying to trouble shoot. The transmission shifts fine and smooth (especially after cleaning the MAF). The fluid is still pink / red and looks full after driven. When I accelerate from a full stop, even if I floor it, the Blazer shifts fine. It isn't until I slow down to about 10mph and press the accelerator that I notice a clank or clunk. If I baby it I don't notice anything.

No SES or codes. Is this a transmission issue, drive shaft issue, or something else?

Placing the vehicle in R usually produces a little more kick then I expect (almost as though 4WD is engaged). Drive is better. The 4x4 won't engage for some reason but it's in 2WD at the moment.